Thermally treated polyamic amide aerogel

ABSTRACT

Thermally treated aerogel compositions that include polyamic amides in an amount less than the aerogel compositions that include polyamic amides prior to thermal treatment, and articles of manufacture that include or are manufactured from the aerogel compositions are described.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No.62/594,786, filed Dec. 5, 2017, which is incorporated herein in itsentirety without disclaimer.

BACKGROUND OF THE INVENTION A. Field of the Invention

The present disclosure relates to the field of aerogels. In particular,the invention concerns thermally treated polyimide aerogels having apolymeric matrix that includes a polyamic amide polymer in an amount ofless than 5 wt. % based on the total weight of the aerogel.

B. Description of Related Art

A gel by definition is a spongelike, three-dimensional solid networkwhose pores are filled with another non-gaseous substance, such as aliquid. The liquid of the gel is not able to diffuse freely from the gelstructure and remains in the pores of the gel. Drying of the gel thatexhibits unhindered shrinkage and internal pore collapse during dryingprovides materials commonly referred to as xerogels.

By comparison, a gel that dries and exhibits little or no shrinkage andinternal pore collapse during drying can yield an aerogel. An aerogel isa light weight material having a relatively low density and highporosity. Aerogels are used in a wide variety of applications such asbuilding and construction, aerospace, catalysts, insulation, sensors,thickening agents, and the like. Aerogels made from organic polymers(e.g., polyimides or silica/polyimide blends) provide lightweight,low-density structures; however, they tend to have lower glasstransition temperatures and degrade at higher temperatures (>150° C.).Attempts to improve the thermal properties of polymer aerogels haveincluded cross-linking tri, tetra, or poly-functional units in thestructure. Although cross-linked polymer aerogels (e.g., polyimideaerogels) can have some acceptable mechanical properties, they typicallysuffer from poor flexibility and can be difficult to manufacture,reprocess, or recycle. The lack of manufacturability and recyclabilitycan have a negative impact on production scale-up, large scalemanufacturing, conformation to irregular surfaces, or maintainingintegrity in dynamic conditions.

Recent efforts to improve upon the flexibility of aerogels, while stillmaintaining good thermal and mechanical properties, have been focused onmodifying the polymers used to create the aerogel matrix. For example,U.S. Pat. No. 9,109,088 to Meader et al., discloses cross-linkedpolyimide aerogels that attempt to impart bulk flexibility by usingflexible linking groups in the polymer backbone. U.S. Pat. No. 9,206,298to Rodman et al., suggests that specific properties of polyimidepolymers, such as flexibility, can be influenced by incorporatingcertain compounds into the polyimide polymer without the formulation ofcovalent bonds. However, the resultant properties of the non-covalentlylinked compounds can be difficult to predict. For instance,non-covalently linked compounds in the polymer matrix can aggregate,which can affect homogeneity, mechanical properties, and otherproperties of the final aerogel.

Despite the foregoing, the above mentioned aerogels can still sufferfrom brittleness, poor thermal stability, and/or complicatedmanufacturing steps.

SUMMARY OF THE INVENTION

A discovery has been made that provides a solution to at least some ofthe aforementioned problems associated with polyimide aerogels. Thediscovery is premised on forming an aerogel containing a polyamic amidepolymer within its polymer matrix and subsequently thermally treatingthe aerogel to reduce the amount of polyamic amide polymer in thepolymer matrix of the aerogel. Notably, an aerogel containing at least 5wt. % of a polyamic amide polymer within its polymer matrix can besubsequently thermally treating the aerogel to reduce the amount ofpolyamic amide polymer to less than 5 wt. %, preferably 0.01 wt. % up to4.95 wt. %. A benefit of thermal treatment is that off-gassing can bereduced or avoided when the aerogel is further processed, handled,and/or otherwise manufactured or incorporated into a desired endproduct. In preferred instances, the aerogel prior to thermal treatmentcan have 5 wt. % to 25 wt. % of the polyamic amide polymer, which canthen be converted into polyimide polymer such that the resulting aerogelhas at least 50 wt. %, 60 wt. %, 70 wt. %, 80 wt. %, 90 wt. %, 95 wt. %or more of polyimide and 0.01 wt. %, 1 wt. %, 2 wt. %, 3 wt. %, 4 wt. %,4.95 wt. % or less of polyamic amide polymer. The resulting thermallytreated polyamic amide aerogels of the present invention can have highbranching and little to no crosslinking. Further, it was surprisinglyfound that the low levels of polyamic amide polymer in the aerogels ofthe present invention can contribute to the aerogels higher flexibility,higher thermal stability, and/or lower thermal conductivity whencompared with existing polyimide-based aerogels. The disclosed aerogelsare stable and are able to resist browning at 330° C. In addition, thepresence of polyamic amide in the polymer matrices of the aerogels ofthe present invention are easier to manufacture and/or recycle. By wayof example, the methods of producing the polyamic amide aerogels of thepresent invention can eliminate or reduce the need for costly reagentsand complex manufacturing steps, which are difficult to control.

Still further, and in certain non-limiting aspects, the polymericmatrices of the aerogels of the present invention can include macropores(pores having a size of greater than 50 nanometers (nm) in diameter),mesopores (pores having a size of 2 nm to 50 nm in diameter), ormicropores (pores having a size of less than 2 nm in diameter), or anycombination of such pores (e.g., macropores and mesopores, macroporesand micropores, macropores, mesopores, and micropores, or mesopores andmicropores). In certain preferred embodiments, the aerogels of thepresent invention include macropores. It is believed that the presenceof macropores can further help facilitate the manufacturing of theaerogels, as macropores are larger and less likely to collapse duringthe drying stage of manufacturing when compared with micropores and/ormesopores. This can result in a more economically efficient and lesscomplicated drying process, thereby allowing for a more commerciallyscalable process when compared with known mesoporous and/or microporousstructured aerogels. Additionally, the presence of macropores mayimprove any one of or all of the flexibility, strength, gas permeation,and/or the strength to density ratio of the formed aerogel. In morepreferred instances, the average pore size of the porous aerogelmatrices of the present invention is greater than 50 nm to 5000 nm indiameter, preferably 100 nm to 2000 nm in diameter, more preferably 500nm to 2000 nm in diameter, even more preferably 1000 nm to 1400 nm indiameter, still more preferably 1100 nm to 1300 nm in diameter, and mostpreferably about 1200 nm in diameter. Additionally, and in somepreferred embodiments, the majority (e.g., more than 50%) of the porevolume in the aerogels of the present invention can be made up frommacropores. In even further instances, over 55%, 60%, 70%, 80%, 90%,95%, 99%, or 100% of the pore volume of the aerogels can be made up ofmacropores. In instances where less than 100% of the pore volume is madeup of macropores, such aerogels can also include mesopores and/ormicropores. This porous architecture along with the incorporation of lowlevels (i.e., less than 5 wt. %) of the aforementioned polyamic amidepolymers into the aerogels is believed to contribute to the improvedmechanical, thermal, manufacturability, and/or recyclability propertiesof the aerogels of the present invention. Even further, the low levelsof the polyamic amide polymers can reduce or avoid off-gassing that mayoccur during further handling of the aerogels, processing of theaerogels, manufacturing of desired end products from the aerogels,and/or incorporation of the aerogels into desired end products.

In one embodiment of the present invention there is disclosed athermally treated polyamic amide aerogel including a polyamic amidepolymer in an amount less than the aerogel prior to thermal treating. Byway of example, a polyamic amide aerogel can have at least 5 wt. %polyamic amide and upon thermal treatment the polyamic amide content canbe reduced by at least 15% or at least 50 wt. % (e.g., to about 4.95 wt.% or less). The thermally treated aerogel can include an open-cellstructured polymer matrix that includes the polyamic amide polymer andpolyimide polymer with a majority of the matrix comprising polyimidepolymer. The polyamic amide polymer can have a repeating structural unitof:

where X can be a first organic group having at least two carbon atoms, Ycan be a second organic group having at least two carbon atoms, and Zand Z′ can each independently be a nitrogen containing hydrocarboncompound that includes at least one secondary nitrogen or a hydroxylgroup. In some instances, the above polyamic amide polymer can be 2 to2000 repeating units in length. Z and Z′ can be the same or different.In some instance, Z is a nitrogen containing hydrocarbon compound and Z′is a hydroxyl group. In one aspect, Z can be a substituted or anunsubstituted cyclic compound, a substituted or an unsubstitutedaromatic compound, or combinations thereof and Z′ can be a hydroxylgroup. In another aspect, Z can further include at least one tertiarynitrogen. By way of example, Z can be an imidazole or a substitutedimidazole, a triazole or a substituted triazole, a tetrazole orsubstituted tetrazole, a purine or a substituted purine, a pyrazole or asubstituted pyrazole, or combinations thereof, and, in some instances,the secondary and tertiary nitrogen atoms are separated by at least onecarbon atom. In a one aspect, Z has the following general structure:

where R₃, R₄, and R₅ can each individually be a hydrogen (H) atom, analkyl group, or a substituted alkyl group, an aromatic group or asubstituted aromatic group, or R₄, and R₅ come together with other atomsto form a fused ring structure. In some instances, the aforementionedalkyl group or substituted alkyl group can have 1 to 12 carbon atoms, 2to 6 carbon atoms, 3 to 8 carbon atoms, 5 to 12 carbon atoms, preferably1 to 6 carbon atoms. In other instances, R₃ can be a methyl group or anethyl group, and R₄ and R₅ can be H atoms, an alkyl group, or asubstituted alkyl group. In some aspects, R₃ can be a methyl group, andR₄ and R₅ are H atoms. R₃ can be an ethyl group, and R₄ and R₅ are eachindividually a H, an alkyl group, or a substituted alkyl, preferably, R₄is a methyl group and R₅ is a H atom. In some instances, the aerogel ofthe present invention can include at least 5 wt. % of the polyamic amidepolymer based on the total weight of the polymer aerogel. In anotheraspect, the thermally treated aerogels of the present invention can haveany one, any combination of, or all of the following characteristics:(1) a density of 0.05 g/cm³ to 0.35 g/cm³; (2) a porosity of at least50, 60, 70, 80, or 90%, preferably at least 85%, and more preferably 85%to 95%; and/or (3) a tensile strength of 100 psi to 2500 psi (0.69 MPato 17.23 MPa) and an elongation of 0.1% to 50%, at room temperature asmeasured according to ASTM D882-02; (4) a compression strength of 10 psito 500 psi (0.069 MPa to 3.45 MPa) at 10% strain at room temperature asmeasured according to ASTM D1621-12; (5) or combinations thereof.

In another embodiment of the present invention, the thermally treatedaerogel further includes a repeating structural unit of:

where Y can be hydroquinone dianhydride;3,3′,4,4′-biphenyltetracarboxylic dianhydride; pyromellitic dianhydride;3,3′,4,4′-benzophenone-tetracarboxylic dianhydride; 4,4′-oxydiphthalicanhydride; 3,3′,4,4′-diphenylsulfone-tetracarboxylic dianhydride;4,4′-(4,4′-isopropylidenediphenoxy)bis(phthalic anhydride);2,2-bis(3,4-dicarboxyphenyl)propane dianhydride;4,4′-(hexafluoroisopropylidene)diphthalic anhydride;bis(3,4-dicarboxyphenyl)sulfoxide dianhydride; polysiloxane-containingdianhydride; 2,2′,3,3′-biphenyltetracarboxylic dianhydride;2,3,2′,3′-benzophenonetetraearboxylic dianhydride;3,3′,4,4′-benzophenonetetraearboxylic dianhydride;naphthalene-2,3,6,7-tetracarboxylic dianhydride;naphthalene-1,4,5,8-tetracarboxylie dianhydride; 4,4′-oxydiphthalicdianhydride; 3,3′,4,4′-biphenylsulfone tetracarboxylic dianhydride;3,4,9,10-perylene tetracarboxylic dianhydride;bis(3,4-dicarboxyphenyl)sulfide dianhydride;bis(3,4-dicarboxyphenyl)methane dianhydride;2,2-bis(3,4-dicarboxyphenyl)propane dianhydride;2,2-bis(3,4-dicarboxyphenyl)hexafluoropropene;2,6-dichloronaphthalene-1,4,5,8-tetracarboxylic dianhydride;2,7-dichloronapthalene-1,4,5,8-tetracarboxylic dianhydride;2,3,6,7-tetrachloronaphthalene-1,4,5,8-tetracarboxylic dianhydride;phenanthrene-8,9,10-tetracarboxylic dianhydride;pyrazine-2,3,5,6-tetracarboxylic dianhydride;benzene-1,2,3,4-tetracarboxylic dianhydride;thiophene-2,3,4,5-tetracarboxylic dianhydride; or combinations thereof.In some instances, the above polyimide polymer can be 2 to 2000repeating units in length. In one aspect, the thermally treated aerogelcan include at least one copolymer including two repeating structuralunits of:

where in and n are an average number of repeat units per chain rangingfrom 1 to 2000. In another aspect, the copolymer can be a branchedcopolymer and the aerogel includes an open-cell structure.

Also disclosed is a method of making aerogels of the present invention.The method can include (a) providing at least one diamine compound to asolvent to form a solution; (b) providing at least one dianhydridecompound to the solution of step (a) under conditions sufficient to forma polyamic acid solution (e.g., agitating at room temperature for adesired amount of time); (c) providing a secondary amine to the polyamicacid solution; (d) subjecting the mixture of step (c) to conditionssuitable (e.g., agitating at room temperature or heating up to 100° C.)to produce a polymer matrix solution including a polyamic amide; (e)subjecting the polymer matrix solution to conditions sufficient to forman aerogel having a polyamic amide polymer; and (f) thermally treatingthe polyamic amide polymer aerogel at a temperature sufficient to lowerthe amount of polyamic amide in the aerogel. Step (f) conditions caninclude heating the aerogel at a temperature of 275° C. to 550° C., 280°C. to 350° C., 290° C. to 350° C. or about 300° C. to produce athermally treated polyimide aerogel under an inert atmosphere. In someembodiments, the aerogel is heated in the presence of air and/or undervacuum. The method can further include heating the thermally treatedpolyimide aerogel, preferably under vacuum conditions (e.g., heating at225° C. to 310° C. under vacuum under an argon atmosphere). In someembodiments, the thermally treated aerogel can be heated at atemperature 225° C. to 310° C. under a gas flow (e.g., an air flow,inert gas flow, etc.). Further heat treatment (e.g., drying) can removeany compounds not chemically bound to the polymer matrix.

The secondary amine of the method can be a substituted or anunsubstituted cyclic amine, a substituted or an unsubstituted aromaticamine, or combinations thereof. In some aspects, the secondary amine caninclude at least one secondary nitrogen and at least one tertiarynitrogen (e.g., an imidazole or a substituted imidazole, a triazole or asubstituted triazole, a tetrazole or substituted tetrazole, a purine ora substituted purine, a pyrazole or a substituted pyrazole, orcombinations thereof). In some instances, the nitrogen atoms areseparated by at least one carbon atom. In certain aspects, the secondaryamine in step (c) has the following general structure:

where R₃, R₄, and R₅ are defined above. In further aspects, step (d) ofthe method includes providing a dehydrating agent prior to, during, orafter adding the secondary amine. Step (e) conditions can include:(e)(1) forming a gel from the solution that includes the solvent as aliquid phase; and (e)(2) removing the solvent from the gel to form theaerogel, such as through drying the gel. Forming a gel can include theaddition of a sufficient amount of a chemical curing agent forsufficient amounts of time to form the gel. In some instances, and afterthe gel formation step but prior to the drying of the gel step, asolvent exchange step can be performed where the initial solvent isreplaced with a second solvent. Multiple solvent exchange steps can beperformed. In preferred instances, the second solvent can be morevolatile than the first solvent, which can make the drying step moreefficient and which can reduce or prevent collapse of the gel matrixduring drying. Non-limiting examples of drying the gel to form theaerogel are described throughout this specification and incorporatedinto this paragraph by reference. Some examples of drying steps includesubcritical drying, supercritical drying, or evaporative air drying, orany combination thereof. Even further, the method can include (i)subjecting the gel to conditions sufficient to freeze the solvent toform a frozen material, and (ii) subjecting the frozen material to asubcritical drying step sufficient to form an open-cell structure.

In instances where there is a desire to incorporate macropores into thepolymeric matrix of any one of the aerogels of the present invention,such macropores can be formed by selecting processing conditions thatfavor the formation of macropores versus mesopores and/or micropores.The amount of macropores can be adjusted by implementing any one of, anycombination of, or all of the following variables: (1) thepolymerization solvent; (2) the polymerization temperature; (3) thepolymer molecular weight; (4) the molecular weight distribution; (5) thecopolymer composition; (6) the amount of branching; (7) the amount ofcrosslinking; (8) the method of branching; (9) the method ofcrosslinking; (10) the method used in formation of the gel; (11) thetype of catalyst used to form the gel; (12) the chemical composition ofthe catalyst used to form the gel; (13) the amount of the catalyst usedto form the gel; (14) the temperature of gel formation; (15) the type ofgas flowing over the material during gel formation; (16) the rate of gasflowing over the material during gel formation; (17) the pressure of theatmosphere during gel formation; (18) the removal of dissolved gassesduring gel formation; (19) the presence of solid additives in the resinduring gel formation; (20) the amount of time of the gel formationprocess; (21) the substrate used for gel formation; (22) the type ofsolvent or solvents used in each step of the solvent exchange process;(23) the composition of solvent or solvents used in each step of thesolvent exchange process; (24) the amount of time used in each step ofthe solvent exchange process; (25) the dwell time of the part in eachstep of the solvent exchange process; (26) the rate of flow of thesolvent exchange solvent; (27) the type of flow of the solvent exchangesolvent; (28) the agitation rate of the solvent exchange solvent; (29)the temperature used in each step of the solvent exchange process; (30)the ratio of the volume of solvent exchange solvent to the volume of thepart; (31) the method of drying; (32) the temperature of each step inthe drying process; (33) the pressure in each step of the dryingprocess; and/or (34) the solvents used in each step of the dryingprocess. In one preferred and non-limiting aspect, the formation ofmacropores versus smaller mesopores and micropores can be primarilycontrolled by controlling the polymer/solvent dynamics during gelformation. By doing so, the pore structure can be controlled, and thequantity and volume of macroporous, mesoporous, microporous cells can becontrolled. In one instance, this can be done by adding a curing agentto the solution to reduce the solubility of polymers formed in thesolution and to form macropores in the gel matrix, the formed macroporescontaining liquid from the solution. For example, a curing additive thatreduces the resultant polymer solubility, such as1,4-diazabicyclo[2.2.2]octane, can produce a polymer matrix containing ahigher number of macropores compared to another curing additive thatimproves the resultant polyimide solubility, such as triethylamine. Inanother example, using the same dianhydride such asbiphenyl-tetracarboxylic acid dianhydride (BPDA), but increasing theratio of rigid amines incorporated into the polymer backbone such asp-phenylenediamine (p-PDA) as compared to more flexible diamines such as4,4′-oxydianiline (ODA), the formation of macropores as compared tosmaller mesopores and micropores can be controlled.

The aerogel of the present invention can be included in articles ofmanufacture/desired end products. Articles of manufacture can be a thinfilm, a monolith, a wafer, a blanket, a core composite material, asubstrate for radiofrequency antenna, a substrate for a sunshield, asubstrate for a sunshade, a substrate for radome, insulating materialfor oil and/or gas pipeline, insulating material for liquefied naturalgas pipeline, insulating material for cryogenic fluid transfer pipeline,insulating material for apparel, insulating material for aerospaceapplications, insulating material for buildings, cars, and other humanhabitats, insulating material for automotive applications, insulationfor radiators, insulation for ducting and ventilation, insulation forair conditioning, insulation for heating and refrigeration and mobileair conditioning units, insulation for coolers, insulation forpackaging, insulation for consumer goods, vibration dampening, wire andcable insulation, insulation for medical devices, support for catalysts,support for drugs, pharmaceuticals, and/or drug delivery systems,aqueous filtration apparatus, oil-based filtration apparatus, andsolvent-based filtration apparatus, or any combination thereof.Preferably, the article of manufacture is an antenna, a sunshield orsunscreen, a radome, a blanket, or a filter.

The aerogel of the present invention can be used to filter a fluid inneed thereof. A filtration method using the aerogel of the presentinvention can include contacting a feed fluid with the aerogel of thepresent invention under conditions sufficient to remove at least aportion of the impurities and/or desired substances from the feed fluidand produce a filtrate. In one aspect, the feed fluid is a liquid, agas, a supercritical fluid, or a mixture thereof. The feed fluid caninclude water or alternatively can be a non-aqueous liquid. When thefeed fluid is a non-aqueous liquid, it can be an oil, a solvent, orcombinations thereof. In a specific aspect, the feed fluid is a solventand the solvent can be an organic solvent. In another specific aspect,the feed fluid is an emulsion and the emulsion can be a water-oilemulsion, an oil-water emulsion, a water-solvent emulsion, asolvent-water emulsion, an oil-solvent emulsion, or a solvent-oilemulsion. The feed fluid can also be a biological fluid and thebiological fluid can be blood, plasma, or both. Additionally, the feedfluid can be a gas and the gas can include air, nitrogen, oxygen, aninert gas, or mixtures thereof. The goal of the method of filtering afluid using the disclosed aerogels is to obtain a filtrate that issubstantially free of impurities and/or a desired substance. In anotherembodiment, a filtration system is disclosed that includes (a) anaerogel of the present invention, and (b) a separation zone in fluidcommunication with the aerogel, a feed fluid and a filtrate.

In the context of the present invention 51 embodiments are described.Embodiment 1 is a method of making, a thermally treated polyamic amideaerogel, the method comprising: (a) providing at least one diaminecompound to a solvent to form a solution; (b) providing at least onedianhydride compound to the solution of step (a) under conditionssufficient to form a polyamic acid solution; (c) providing a secondaryamine to the polyamic acid solution; (d) subjecting the solution of step(c) to conditions suitable to produce a polymer matrix solutioncomprising a polyamic amide; (e) subjecting the polymer matrix solutionto conditions sufficient to form an aerogel comprising an open-cellstructured polymer matrix having a polyamic amide; and (f) thermallytreating the (e) polyamic amide aerogel at a temperature sufficient tolower the amount of the polyamic amide in the aerogel. Embodiment 2 isthe method of embodiment 1, wherein step (f) conditions comprise heatingthe aerogel at a temperature of 275° C. to 550° C., or 290° C. to 500°C., or 300° C. to 350° C., to produce a thermally treated polyimideaerogel. Embodiment 3 is the method of any one of embodiments 1 to 2,wherein step (f) is performed under an inert atmosphere or in air.Embodiment 4 is the method of any one of embodiments 1 to 3, furthercomprising subjecting the thermally treated polyimide aerogel to asecond temperature cycle, under vacuum or in air to remove compounds notchemically bound to the polymer matrix. Embodiment 5 is the method ofany one of embodiments 1 to 4, wherein the aerogel includes at least0.01 wt. % and up to 4.95 wt. % of the polyamic amide polymer,preferably 1 wt. % to 3 wt. %. Embodiment 6 is the method of any one ofembodiments 1 to 5, wherein thermally treating reduces the amount ofpolyamic amide in the aerogel by at least 15%, or at least 50%.Embodiment 7 is the method of any one of embodiments 1 to 6, wherein thesecondary amine is a substituted or an unsubstituted cyclic amine, asubstituted or an unsubstituted aromatic amine, or combinations thereof.Embodiment 8 is the method of any one of embodiments 1 to 7, whereinsecondary amine further comprises at least one secondary nitrogen and atleast one tertiary nitrogen. Embodiment 9 is the method of embodiment 8,wherein the secondary amine is imidazole or a substituted imidazole, atriazole or a substituted triazole, a tetrazole or substitutedtetrazole, a purine or a substituted purine, a pyrazole or a substitutedpyrazole, or combinations thereof. Embodiment 10 is the method ofembodiment 9, wherein the nitrogen atoms are separated by at least onecarbon atom. Embodiment 11 is the method of any one of embodiments 1 to10, wherein the secondary amine in step (c) has the following generalstructure:

where R₃, R₄, and R₅ are each individually a hydrogen, an alkyl group,or a substituted alkyl group, or an aromatic group or a substitutedgroup, or R₄, and R₅ come together with other atoms to form a cyclicstructure. Embodiment 12 is the method of embodiment 11, wherein thealkyl group has 1 to 12 carbon atoms, 2 to 6 carbon atoms, 3 to 8 carbonatoms, 5 to 12 carbon atoms, preferably 1 to 6 carbon atoms. Embodiment13 is the method of any one of embodiments 11 to 12, wherein R₃ is amethyl group or an ethyl group, and R₄ and R₅ are H atoms, an alkylgroup, or a substituted alkyl. Embodiment 14 is the method of embodiment13, wherein R₃ is a methyl group and R₄ and R₅ are H atoms. Embodiment15 is the method of embodiment 14, wherein R₃ is an ethyl group and R₄and R₅ are each individually a H atom, an alkyl group, or a substitutedalkyl, preferably, R₄ is a methyl group and R₅ is a H atom. Embodiment16 is the method of any one of embodiments 1 to 15, wherein step (d)comprises providing a dehydrating agent prior to, during, or after,adding the secondary amine. Embodiment 17 is the method of any one ofembodiments 1 to 16, wherein step (e) comprises forming a gel from thesolution and removing the solvent from the gel. Embodiment 18 is themethod of embodiment 17, comprising subjecting the gel to a drying stepto remove the solvent. Embodiment 19 is the method of embodiment 18,wherein the drying step comprises supercritical drying, subcriticaldrying, thermal drying, evaporative air drying, vacuum drying, or anycombination thereof. Embodiment 20 is the method of embodiment 19,wherein drying comprises evaporative air drying. Embodiment 21 is themethod of embodiment 20, wherein the drying step comprises: (i)subjecting the gel to conditions sufficient to freeze the solvent toform a frozen material; and (ii) subjecting the frozen material to asubcritical drying step sufficient to form an open-cell structure.Embodiment 22 is the method of any one of embodiments 1 to 21, furthercomprising subjecting the gel to at least one solvent exchange with adifferent solvent prior to drying the gel. Embodiment 23 is the methodof embodiment 22, wherein at least one solvent exchange is performedwith acetone.

Embodiment 24 is a thermally treated polyamic amide aerogel comprisingan open-cell structured polymer matrix that includes a polyamic amidepolymer in an amount less than the polyamic amide polymer prior tothermal treatment. Embodiment 24 is the thermally treated polyamic amideaerogel of embodiment 24, wherein the aerogel, when exposed to heat,does not produce a gas. Embodiment 26 is the aerogel of any one ofembodiments 24 to 25, wherein the polyamic amide polymer in the matrixhas a repeating structural unit of:

where X is a first organic group having at least two carbon atoms, Y isa second organic group having at least two carbon atoms, and Z and Z′are each independently a hydroxyl group or a nitrogen containinghydrocarbon compound comprising at least one secondary nitrogen.Embodiment 27 is the aerogel of embodiment 26, wherein Z is asubstituted or an unsubstituted cyclic compound, a substituted or anunsubstituted aromatic compound, or combinations thereof. Embodiment 28is the aerogel of any one of embodiments 26 to 27, wherein Z furthercomprises at least one tertiary nitrogen. Embodiment 29 is the aerogelof embodiment 28, wherein Z is an imidazole or a substituted imidazole,a triazole or a substituted triazole, a tetrazole or substitutedtetrazole, a purine or a substituted purine, a pyrazole or a substitutedpyrazole, or combinations thereof. Embodiment 30 is the aerogel ofembodiment 28, wherein the secondary and tertiary nitrogen atoms areseparated by at least one carbon atom. Embodiment 31 is the aerogel ofembodiment 30, wherein Z has the following general structure:

where R₃, R₄, and R₅ are each individually a hydrogen (H) atom, an alkylgroup, or a substituted alkyl group, an aromatic group or a substitutedaromatic group, or R₄, and R₅ come together with other atoms to form afused ring structure. Embodiment 32 is the aerogel of embodiment 31,wherein the alkyl group or a substituted alkyl group has 1 to 12 carbonatoms, 2 to 6 carbon atoms, 3 to 8 carbon atoms, 5 to 12 carbon atoms,preferably 1 to 6 carbon atoms. Embodiment 33 is the aerogel of any oneof embodiments 31 to 32, wherein R₃ is a methyl group or an ethyl groupand R₄ and R₅ are H atoms, an alkyl group, or a substituted alkyl group.Embodiment 34 is the aerogel of embodiment 33, wherein R₃ is a methylgroup, and R₄ and R₅ are H atoms. Embodiment 35 is the aerogel ofembodiment 33, wherein R₃ is an ethyl group and R₄ and R₅ are eachindividually a H atom, an alkyl group, or a substituted alkyl,preferably, R₄ is a methyl group and R₅ is a H atom. Embodiment 36 isthe aerogel of any one of embodiments 26 to 35, wherein Z′ is a hydroxylgroup and Z is an imidazole group. Embodiment 37 is the aerogel of anyone of embodiments 26 to 36, further comprising a polyimide polymer.Embodiment 38 is the aerogel of embodiment 37, wherein the polyimidepolymer has a repeating structural unit of:

Embodiment 39 is the aerogel of any one of embodiments 26 to 38, whereinthe Y is derived from hydroquinone dianhydride;3,3′,4,4′-biphenyltetracarboxylic dianhydride; pyromellitic dianhydride;3,3′,4,4′-benzophenone-tetracarboxylic dianhydride; 4,4′-oxydiphthalicanhydride; 3,3′,4,4′-diphenylsulfone-tetracarboxylic dianhydride;4,4′-(4,4′-isopropylidenediphenoxy)bis(phthalic anhydride);2,2-bis(3,4-dicarboxyphenyl)propane dianhydride;4,4′-(hexafluoroisopropylidene)diphthalic anhydride;bis(3,4-dicarboxyphenyl)sulfoxide dianhydride; polysiloxane-containingdianhydride; 2,2′,3,3′-biphenyltetracarboxylic dianhydride;2,3,2′,3′-benzophenonetetraearboxylic dianhydride;3,3′,4,4′-benzophenonetetraearboxylic dianhydride;naphthalene-2,3,6,7-tetracarboxylic dianhydride;naphthalene-1,4,5,8-tetracarboxylie dianhydride; 4,4′-oxydiphthalicdianhydride; 3,3′,4,4′-biphenylsulfone tetracarboxylic dianhydride;3,4,9,10-perylene tetracarboxylic dianhydride;bis(3,4-dicarboxyphenyl)sulfide dianhydride;bis(3,4-dicarboxyphenyl)methane dianhydride;2,2-bis(3,4-dicarboxyphenyl)propane dianhydride;2,2-bis(3,4-dicarboxyphenyl)hexafluoropropene;2,6-dichloronaphthalene-1,4,5,8-tetracarboxylic dianhydride;2,7-dichloronapthalene-1,4,5,8-tetracarboxylic dianhydride;2,3,6,7-tetrachloronaphthalene-1,4,5,8-tetracarboxylic dianhydride;phenanthrene-8,9,10-tetracarboxylic dianhydride;pyrazine-2,3,5,6-tetracarboxylic dianhydride;benzene-1,2,3,4-tetracarboxylic dianhydride;thiophene-2,3,4,5-tetracarboxylic dianhydride; or combinations thereof.Embodiment 40 is the aerogel of any one of embodiments 26 to 39, whereinthe aerogel comprises at least one copolymer comprising two repeatingstructural units of:

where m and n are average number of repeat units per chain ranging from1 to 2000. Embodiment 41 is the aerogel of embodiment 40, wherein thecopolymer is a branched copolymer. Embodiment 42 is the aerogel of anyone of embodiments 26 to 41, wherein the aerogel includes at 0.01 wt. %to up to 6 wt. % of the polyamic amide polymer. Embodiment 43 is theaerogel of any one of embodiments 26 to 42, wherein the polymer matrixhas an average pore size of greater than 50 nanometers (nm) to 5000 nmin diameter, preferably 100 nm to 2000 nm in diameter, more preferably500 nm to 2000 nm in diameter, even more preferably 1000 nm to 1400 nmin diameter, still more preferably 1100 nm to 1300 nm in diameter, andmost preferably about 1200 nm in diameter. Embodiment 44 is the aerogelof embodiment 43, wherein the polymer matrix has an average pore size of1000 nm to 1400 nm in diameter. Embodiment 45 is the aerogel ofembodiment 44, wherein the polymer matrix has an average pore size of1100 nm to 1300 nm, preferably about 1200 nm in diameter. Embodiment 46is an article of manufacture comprising the aerogel of any one ofembodiments 26 to 45. Embodiment 47 is the article of manufacture ofembodiment 46, wherein the article of manufacture is a thin film,monolith, wafer, blanket, core composite material, a substrate forradiofrequency antenna, substrate for a sunshield, a substrate for asunshade, a substrate for radome, insulating material for oil and/or gaspipeline, insulating material for liquefied natural gas pipeline,insulating material for cryogenic fluid transfer pipeline, insulatingmaterial for apparel, insulating material for aerospace applications,insulating material for buildings, cars, and other human habitats,insulating material for automotive applications, insulation forradiators, insulation for ducting and ventilation, insulation for airconditioning, insulation for heating and refrigeration and mobile airconditioning units, insulation for coolers, insulation for packaging,insulation for consumer goods, vibration dampening, wire and cableinsulation, insulation for medical devices, support for catalysts,support for drugs, pharmaceuticals, and/or drug delivery systems,aqueous filtration apparatus, oil-based filtration apparatus, andsolvent-based filtration apparatus, or any combination thereof.Embodiment 48 is the article of manufacture of embodiment 47, whereinthe article of manufacture is an antenna. Embodiment 49 is the articleof manufacture of embodiment 47, wherein the article of manufacture is asunshield or sunscreen. Embodiment 50 is the article of manufacture ofembodiment 47, wherein the article of manufacture is a radome.Embodiment 51 is the article of manufacture of embodiment 47, whereinthe article of manufacture is a filter.

The following includes definitions of various terms and phrases usedthroughout this specification.

The term “aerogel” refers to a class of materials that are generallyproduced by forming a gel, removing a mobile interstitial solvent phasefrom the pores, and then replacing it with a gas or gas-like material.By controlling the gel and evaporation system, density, shrinkage, andpore collapse can be minimized. As explained above, aerogels of thepresent invention can include micropores, mesopores, or macropores, orany combination thereof. The amount of micropores, mesopores, and/ormacropores in any given aerogel of the present invention can be modifiedor tuned as desired. In certain preferred aspects, however, the aerogelscan include macropores such that at least 5%, 10%, 15%, 20%, 25%, 30%,35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, or 100%of the aerogel's pore volume can be made up of macropores. In someembodiments, the aerogels of the present invention can have low bulkdensities (about 0.50 g/cm³ or less, preferably about 0.01 to 0.5g/cm³), high surface areas (generally from about 10 to 1,000 m²/g andhigher, preferably about 50 to 1000 m²/g), high porosity (about 80% andgreater, preferably greater than about 85%), and/or relatively largepore volume (more than about 1.0 mL/g, preferably about 1.2 mL/g andhigher).

The presence of mesopores and/or micropores in the aerogels of thepresent invention can be determined by mercury intrusion porosimetry(MIP) and/or gas physisorption experiments. In a preferred instance, theMIP test used in the Examples section can be used to measure themesopores above 5 nm (i.e., American Standard Testing Method (ASTM)D4404-10, Standard Test Method for Determination of Pore Volume and PoreVolume Distribution of Soil and Rock by Mercury Intrusion Porosimetry).In a preferred instance, gas physisorption experiments are used in theExamples section can be used to measure mesopores and/or micropores(ASTM D1993-03(2008) Standard Test Method for PrecipitatedSilica—Surface Area by Multipoint BET Nitrogen).

The terms “impurity” or “impurities” refers to unwanted substances in afeed fluid that are different than a desired filtrate and/or areundesirable in a filtrate. In some instances, impurities can be solid,liquid, gas, or supercritical fluid. In some embodiments, an aerogel canremove some or all of an impurity.

The term “desired substance” or “desired substances” refers to wantedsubstances in a feed fluid that are different than the desired filtrate.In some instances, the desired substance can be solid, liquid, gas, orsupercritical fluid. In some embodiments, an aerogel can remove some orall of a desired substance.

The term “radio frequency (RF)” refers to the region of theelectromagnetic spectrum having wavelengths ranging from 10⁻⁴ to 10⁷ m.

The term “supercritical fluid” refers to any substance at a temperatureand pressure above its critical point. A supercritical fluid can diffusethrough solids like a gas, and dissolve materials like a liquid.Additionally, close to the critical point, small changes in pressure ortemperature result in large changes in density.

The use of the words “a” or “an” when used in conjunction with any ofthe terms “comprising,” “including,” “containing,” or “having” in theclaims, or the specification, may mean “one,” but it is also consistentwith the meaning of “one or more,” “at least one,” and “one or more thanone.”

The terms “about” or “approximately” are defined as being close to asunderstood by one of ordinary skill in the art. In one non-limitingembodiment, the terms are defined to be within 10%, preferably within5%, more preferably within 1%, and most preferably within 0.5%.

The term “substantially” and its variations are defined to includeranges within 10%, within 5%, within 1%, or within 0.5%.

The use of the term “or” in the claims is used to mean “and/or” unlessexplicitly indicated to refer to alternatives only or the alternativesare mutually exclusive, although the disclosure supports a definitionthat refers to only alternatives and “and/or.”

As used in this specification and claim(s), the words “comprising” (andany form of comprising, such as “comprise” and “comprises”), “having”(and any form of having, such as “have” and “has”), “including” (and anyform of including, such as “includes” and “include”) or “containing”(and any form of containing, such as “contains” and “contain”) areinclusive or open-ended and do not exclude additional, unrecitedelements or method steps.

The thermally treated polyamic amide aerogels of the present inventioncan “comprise,” “consist essentially of,” or “consist of” particularingredients, components, compositions, etc. disclosed throughout thespecification. With respect to the transitional phase “consistingessentially of,” in one non-limiting aspect, a basic and novelcharacteristic of the thermally treated polyamic amide aerogels of thepresent invention is that they have improved mechanical and/or thermalproperties due to the presence of a low amount (less than 5 wt. %) ofpolyamic amide polymer in the aerogel matrix. In addition, thermaltreatment of the aerogels of the present invention can reduce or avoidoff-gassing that may occur during further handling of the aerogels,processing of the aerogels, manufacturing of desired end products fromthe aerogels, and/or incorporation of the aerogels into desired endproducts.

Other objects, features and advantages of the present invention willbecome apparent from the following figures, detailed description, andexamples. It should be understood, however, that the figures, detaileddescription, and examples, while indicating specific embodiments of theinvention, are given by way of illustration only and are not meant to belimiting. Additionally, it is contemplated that changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description. Infurther embodiments, features from specific embodiments may be combinedwith features from other embodiments. For example, features from oneembodiment may be combined with features from any of the otherembodiments. In further embodiments, additional features may be added tothe specific embodiments described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Advantages of the present invention may become apparent to those skilledin the art with the benefit of the following detailed description andupon reference to the accompanying drawings.

FIG. 1 is a reaction schematic showing the synthesis of a polyimidepolymer including polyisoimide.

FIG. 2 is a reaction schematic of an embodiment showing the synthesis ofa polyimide polymer including polyamic amide.

FIG. 3 is a reaction schematic of an embodiment showing the formation ofpolyimide polymers.

FIG. 4 distribution of pore diameters of an aerogel monolith of thepresent invention obtained by freeze drying.

FIG. 5 distribution of pore diameters of an aerogel monolith of thepresent invention obtained by freeze drying.

FIG. 6 distribution of pore diameters of an aerogel monolith of thepresent invention obtained by freeze drying.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof are shown by way ofexample in the drawings and may herein be described in detail. Thedrawings may not be to scale.

DETAILED DESCRIPTION OF THE INVENTION

A discovery has been made that provides a thermally-treated polyamicamide aerogel with improved mechanical and thermal properties as well asimproved manufacturability and processability over conventionalpolyimide aerogels. Without wishing to be bound by theory, it isbelieved that low levels (i.e., less than 5 wt. %) of polyamic amidepolymer present in the aerogel polymer matrix can help contribute tothese improved characteristics. Still further, thermal treatment of theaerogels of the present invention can reduce or avoid off-gassing thatmay occur during further handling of the aerogels, processing of theaerogels, manufacturing of desired end products (i.e., articles ofmanufacture) from the aerogels, and/or incorporation of the aerogelsinto desired end products. These and other non-limiting aspects of thepresent invention are discussed in further detail in the followingsections.

A. Polyamic Amide Polymer

In a particular embodiment, the aerogel of the current inventionincludes a polymeric matrix having a polyamic amide polymer. Thepresence of the polyamic amide polymer surprisingly provides the aerogelwith many improved properties over conventional polyimide aerogels.These improved properties can be obtained with low levels (i.e., lessthan 5 wt. %) of the polyamic amide polymer present in the aerogels. Insome embodiments, the polymer aerogels contain little to no polyisoimidebyproduct in the polymer matrix. In general, polyamic amide polymersinclude two amides that are part of the polymer backbone, and at leasttwo additional amides that are not part of the polymer backbone. The atleast two amides not part of the polymer backbone are free to rotate andinteract with functional groups within and not within the polymerbackbone. This structural arrangement may help to reduce the linearityand stiffness of the polymer backbone in a way to benefit theflexibility of the resultant aerogel while retaining or even increasingmechanical and thermal properties. The amides not part of the polymerbackbone can also be variably functionalized with different amines toprovide further opportunity for chemical interactions and theinstallation of further functionality to further affect aerogelproperties. Similar to polyimide polymer, polyamic amide polymer can beconsidered an AA-BB type polymer because usually two different classesof monomers are used to produce the polyamic amide polymer. However,polyamic amides are different than polyimides in that the intermediatepolyamic acid derivative can be reacted with a free amine instead ofcyclodehydration to form the polyimide.

Polyamic amides can also be prepared from AB type monomers. For example,an aminodicarboxylic acid monomer can be polymerized to form an AB typeintermediate polyamic acid that can be treated with a free amine undercondition to form a polyamic amide. Monoamines and/or mono anhydridescan be used as end capping agents if desired.

The polyamic amide of the current invention has a repeating structuralunit of:

where X can be a first organic group having at least two carbon atoms, Ycan be a second organic group having at least two carbon atoms, and Zand Z′ can each independently be a nitrogen containing hydrocarboncompound comprising at least one secondary nitrogen or a hydroxyl group.Z and Z′ can be the same or different groups. Z and Z′ can be asubstituted or an unsubstituted cyclic compound, a substituted or anunsubstituted aromatic compound, or combinations thereof. In someinstances, the above polyamic amide polymer can be 2 to 2000 repeatingunits in length. Z and Z′ can also include at least one tertiarynitrogen, and, in some instances, the secondary and tertiary nitrogenatoms are separated by at least one carbon atom. Non-limiting examplesof Z and Z′ compounds include an imidazole or a substituted imidazole, atriazole or a substituted triazole, a tetrazole or substitutedtetrazole, a purine or a substituted purine, a pyrazole or a substitutedpyrazole, or combinations thereof. More specifically, Z and Z′ can havethe following general structure:

where R₃, R₄, and R₅ can be each individually a hydrogen (H) atom, analkyl group, or a substituted alkyl group, an aromatic group or asubstituted aromatic group, or R₄, and R₅ come together with other atomsto form a fused ring structure. In some instances, the imidazole canundergo electrophilic aromatic acylation to bond a carbon atom of theimidazole with the carbonyl carbon bonded to Y. An alkyl group can be astraight or branched chain alkyl having 1 to 20 carbon atoms andincludes, for example, methyl, ethyl, propyl, isopropyl, butyl,isobutyl, secondary butyl, tertiary butyl, pentyl, isopentyl, neopentyl,hexyl, heptyl, octyl, 2-ethylhexyl, 1,1,3,3-tetramethylbutyl, nonyl,decyl, dodecyl, tetradecyl, hexadecyl, octadecyl, and eicosyl. Asubstituted alkyl group can be any of the aforementioned alkyl groupsthat are additionally substituted with an heteroatom, such as a halogen(F, Cl, Br, I), boron, oxygen, nitrogen, sulfur, silicon, etc. Anaromatic group can be any aromatic hydrocarbon group having from 6 to 20carbon atoms of the monocyclic, polycyclic or condensed polycyclic type,and include, for example, phenyl, biphenyl and naphthyl. A substitutedaromatic group can be any of the aforementioned aromatic groups that areadditionally substituted with an heteroatom, such as a halogen (F, Cl,Br, I), boron, oxygen, nitrogen, sulfur, silicon, etc. A fused ringstructure includes, for example, benzimidazole. In some instances, theaforementioned alkyl group or substituted alkyl group has 1 to 12 carbonatoms, 2 to 6 carbon atoms, 3 to 8 carbon atoms, 5 to 12 carbon atoms,preferably 1 to 6 carbon atoms. In other instances, R₃ can be a methylgroup or an ethyl group and R₄ and R₅ are H atoms, an alkyl group, or asubstituted alkyl group. In some aspects, R₃ can be a methyl group, andR₄ and R₅ are H atoms, and, in other aspects, R₃ can be an ethyl groupand R₄ and R₅ are each individually a H, an alkyl group, or asubstituted alkyl, preferably, R₄ is a methyl group and R₅ is a H atom.The polyamic amide can have the following general structure when Z′ isan imidazole or substituted imidazole and Z is a hydroxyl group:

In a particular embodiment, the polyamic amide polymer is:

The polyamic amide polymer can be synthesized by several methods. In onemethod of synthesizing the aromatic polyamic amide polymer, a solutionof the aromatic diamine in a polar aprotic solvent, such asN-methylpyrrolidone (NMP), is prepared. A di-acid monomer, usually inthe form of a dianhydride, is added to this solution, but the order ofaddition of the monomers can be varied. For example, the di-acid monomercan be added first, or the di-acid monomer and the diamine can besimultaneously added. The resulting polycondensation reaction forms apolyamic acid, also referred to as a polyamide acid, which is a polyamicamide precursor. Other polyamic amide precursors are known, includingpolyamic ester, polyamic acid salts, polysilyl esters, andpolyisoimides. Once the polyamic acid or derivative is formed, it can befurther reacted with a nitrogen containing hydrocarbon and dehydrationagent under conditions to form the polyamic amide polymer. The nitrogencontaining hydrocarbon and dehydration agent together or separately maybe present in solution, added during the reaction process, or added in aseparate step as appropriate so the nitrogen containing hydrocarbon canbe incorporated into the polyamic amide polymer by an amidation process.“Amidation” is defined as the conversion of a polyamic amide precursorinto a polyamic amide. In some aspects, the molar ratio of a nitrogencontain hydrocarbon to anhydride or diamine monomer can be from 0.031:1to 128:1, 0.12:1 to 32:1, or specifically from 0.5:1 to 8:1. The molarratio of nitrogen containing hydrocarbon to dehydration agent can befrom 01:1 to 44:1, 0.04:1 to 11:1, or specifically from 0.17:1 to 2.8:1.In general, amidation reactions, such as the reaction between acarboxylic acid and amine to form a amide bond are thermodynamicallyfavorable, but often suffer from a high activation energy due acid-basechemistry between the carboxylic acid and amine. To overcome the highactivation energy, amidation reactions often rely on non-acidicactivation of the acid derivative. Activation can be achieved using adehydration agent. For example, the activated acid derivative can bemixed with an acetic anhydride such as trifluoroacetic anhydride (TFAA)and trifluoroacetic acid (TFA) in toluene. In a preferred embodiment,amidation to form polyamic amide polymer can be achieved using anorganic compound having at least one secondary amine. In one particularinstance, an organic compound having a secondary and a tertiary amine,such as 2-methylimidazole or 2-ethyl-4-methylimidazole can be used. Thedehydration agent can include acetic anhydride, propionic anhydride,n-butyric anhydride, benzoic anhydride, trifluoroacetic anhydride,oxalyl chloride, thionyl chloride, phosphorus trichloride,dicyclohexylcarbodiimide, 1,1′-carbonyldiimidazole (CDI), di-tert-butyldicarbonate (Boc₂O), or combinations thereof. The reaction temperaturescan be determined by a skilled chemist or engineer. In some embodiments,the reaction temperatures of one or more steps can range from 20° C. to150° C., or greater than any one of, equal to any one of, or between anytwo of 20° C., 30° C., 40° C., 50° C., 60° C., 70° C., 80° C., 90° C.,100° C., 110° C., 120° C., 130° C., 140° C., and 150° C.

B. Polyimide Polymer

In another embodiment, the polymeric matrices of the aerogels of thepresent invention can also include a polyimide polymer. In somepreferred instances, the majority of the polymeric matrix is comprisedof a polyimide polymer (e.g., at least 50 wt. %, 60 wt. %, 70 wt. %, 80wt. %, 90 wt. %, 95, wt. % 96, wt. %, 97 wt. %, 98 wt. %, 99 wt. %, 99.5wt. %, or any range or number therein (e.g., 50 wt. % to 99.5 wt. %, 80wt. % to 99.5 wt. %, 90 wt. % to 99.5 wt. %, 90 wt. % to 95 wt. %,etc.). Polyimides polymers can be used in production of aerogels withmany desirable properties. In general, polyimide polymers include anitrogen atom in the polymer backbone, where the nitrogen atom isconnected to two carbonyl carbons, such that the nitrogen atom issomewhat stabilized by the adjacent carbonyl groups. A carbonyl groupincludes a carbon, referred to as a carbonyl carbon, which is doublebonded to an oxygen atom. Polyimides are usually considered an AA-BBtype polymer because usually two different classes of monomers are usedto produce the polyimide polymer. Polyimides can also be prepared fromAB type monomers. For example, an aminodicarboxylic acid monomer can bepolymerized to form an AB type polyimide. Monoamines and/or monoanhydrides can be used as end capping agents if desired.

The polyimide of the current invention can have a repeating structuralunit of:

where X can be a first organic group having at least two carbon atomsand Y can be a second organic group having at least two carbon atoms,where X and Y are defined above. In some instances, the above polyimidepolymer can be 2 to 2000 repeating units in length.

Polyimides may be synthesized by several methods. In one method ofsynthesizing aromatic polyimides, a solution of the aromatic diamine ina polar aprotic solvent, such as N-methylpyrrolidone (NMP), is prepared.A di-acid monomer, usually in the form of a dianhydride, is added tothis solution, but the order of addition of the monomers can be varied.For example, the di-acid monomer can be added first, or the di-acidmonomer and the diamine can be simultaneously added. The resultingpolycondensation reaction forms a polyamic acid, also referred to as apolyamide acid, which is a polyimide precursor. Other polyimideprecursors are known, including polyamic ester, polyamic acid salts,polysilyl esters, and polyisoimides. This process description may beapplicable to one or more polyimide precursor solutions. Alternativelythe polyimide can be formed from the forward or reverse mixing of aminesand anhydrides under appropriate dehydrating conditions and/or catalystswhere the lifetime of the polyamic acid intermediate is very short orpossibly not even detectable. The polyimide polymer is formed by acyclodehydration reaction, also called imidization. “Imidization” isdefined as the conversion of a polyimide precursor into an imide.Alternatively, polyamic acids or other precursors may be converted insolution to polyimides by using a chemical dehydrating agent, catalyst,and/or heat.

C. Highly Branched Non-Crosslinked Aerogel

In some aspects, the present disclosure provides an aerogel thatincludes an open-cell structure and a branched polymer matrix. In someembodiments, the matrix contains less than 5%, less than 4%, less than3%, or less than 2% by weight of crosslinked polymers. The branchedpolymer matrix of the aerogel composition can include less than 1% byweight of crosslinked polymers. In some embodiments, the branchedpolymer matrix of the aerogel composition is not crosslinked.

The characteristics or properties of the final aerogel can be impactedby the choice of monomers used to produce the aerogel. Factors to beconsidered when selecting monomers include the properties of the finalaerogel, such as the flexibility, thermal stability, coefficient ofthermal expansion (CTE), coefficient of hydroscopic expansion (CHE) andany other properties specifically desired, as well as cost. Often,certain important properties of a polymer for a particular use can beidentified. Other properties of the polymer may be less significant, ormay have a wide range of acceptable values; so many different monomercombinations could be used. The aerogel composition of the currentinvention includes a high degree of branching and low degree ofcrosslinking, which has a positive effect the polymers' mechanicalproperties. A highly crosslinked polymer is typically considered athermoset polymer, which is a polymer that has been irreversibly cured.The polymers presented herein display a low degree of crosslinking,thereby more closely resembling a thermoplastic. As such, the polymermay be re-shaped and re-cycled. In some aspects, the current aerogelcomposition includes polyamic amide polymer containing a large amount oftrifunctional, tetrafunctional, or multifunctional monomer, specificallytriamine monomer, yet displays little to no crosslinking.

Other factors to be considered in the selection of monomers include theexpense and availability of the monomers chosen. Commercially availablemonomers that are produced in large quantities generally decrease thecost of producing polymer materials since such monomers are in generalless expensive than monomers produced on a lab scale and pilot scale.Additionally, the use of commercially available monomers can improve theoverall reaction efficiency because additional reactions are notrequired to produce a monomer, which is then incorporated into thepolymer.

The highly branched aerogels of the current invention may contain imideco-monomer units that include relatively rigid molecular structures suchas aromatic/cyclic moieties. These typical structures may often berelatively linear and stiff. The linearity and stiffness of thecyclic/aromatic backbone reduces segmental rotation and allows formolecular ordering which results in lower coefficient of thermalexpansion than many thermoplastic polymers having more flexible chains.In addition, the intermolecular associations of polyimide chains provideresistance to most solvents, which tends to reduce the solubility ofmany typical polyimide polymers in many solvents. In some aspects, theuse of more aliphatic monomers can reduce the stiffness of the aerogel,if desired.

The aerogel composition can include a hyperbranched polymer. Ahyperbranched polymer is a highly branched macromolecule withthree-dimensional dendritic architecture. Hence, the molecular weight ofa hyperbranched polymer is not a sufficient parameter that characterizesthese polymers. Since the number of possible structures becomes verylarge as the polymerization degree of macromolecules increases, there isa need to characterize also this aspect of hyperbranched polymers. Thus,the term degree of branching (DB) was introduced as a quantitativemeasure of the branching perfectness for hyperbranched polymers. Thebranched polyimides of the current aerogels can include a degree ofbranching (DB) of at least 0.2, 0.3, 0.4, 0.5, or more branches perpolyimide polymer chain. In further embodiments, DB may range from 0.2to 10, preferably from 1.2 to 8, or more preferably from 3 to 7. In aparticular embodiment, the degree of branching is 6.3. Alternatively,the DB may range from 0.2 to 5, preferably 0.2 to 1, more preferably 0.2to 0.6, or even more preferably about 0.2 to 0.4, or about 0.32. Inanother aspect, the DB may range from 0.3 to 0.7, 0.4 to 0.6, or about0.51. The DB is represented by the following equation:

$\frac{2Q_{T}}{3 - Q_{T} + {3Q_{M}} - {3p}}$

where p is the extent of reaction, and Q_(T) and Q_(M) are parametersrepresenting the fractions of monofunctional and trifunctional monomersat the beginning of the reaction according to the following equations:

$Q_{T} = \frac{3N_{T}}{N_{M} + {2N_{B}} + {3N_{T}}}$$Q_{M} = \frac{N_{M}}{N_{M} + {2N_{B}} + {3N_{T}}}$

where N_(T), N_(M), and N_(B) are the initial number of trifunctional,monofunctional, and bifunctional monomers, respectively.

The highly branched non-crosslinked aerogels of the current inventioncan be prepared from step-growth polymers. Step-growth polymers are agroup of polymeric chemicals that have many uses and beneficialproperties. Step-growth polymers can be formed via step-growthpolymerization in which bifunctional or multifunctional monomers reactto form first dimers, then trimers, then longer oligomers and eventuallylong chain polymers. Generally, step-growth polymers can have robustmechanical properties including toughness and high temperatureresistance that make them desirable over other polymer types. There arenumerous varieties of step-growth polymers, including, polyamic amides,polyimides, polyurethanes, polyureas, polyamides, phenolic resins,polycarbonates, and polyesters. In one embodiment the aerogels of thecurrent invention include a polyamic amide polymer. In anotherembodiment the aerogels of the current invention include a polyamicamide polymer and a polyimide polymer.

The characteristics or properties of the final polymer are significantlyimpacted by the choice of monomers, which are used to produce thepolymer. Factors to be considered when selecting monomers include theproperties of the final polymer, such as the flexibility, thermalstability, coefficient of thermal expansion (CTE), coefficient ofhydroscopic expansion (CHE) and any other properties specificallydesired, as well as cost. Often, certain important properties of apolymer for a particular use can be identified. Other properties of thepolymer may be less significant, or may have a wide range of acceptablevalues; so many different monomer combinations could be used.

D. Aerogel Polymer Compositions

In certain embodiments the aerogel of the current invention prior tothermal treatment can include a polymer or copolymer having repeatingunits of polyamic amide and polyimide:

where m and n are the average number of repeat units per chain rangingfrom 1 to 2000. In one aspect the average number of m can be 1 to 2000,preferably 10 to 1000, and the average number of n can be 1 to 2000,preferably 10 to 1000. In another aspect the ratio of m:n can be 0.001:1to 1000:1, preferably 0.1:1 to 10:1. After thermal treatment, theaverage number of m and n repeat units per chain can range from 1 to1000. In one aspect the average number of m can be 1 to 2000, preferably10 to 1000, and the average number of n can be 1 to 100, preferably 10to 500. In another aspect, the ratio of m:n can be 1000:1 to 1000:00.1,preferably 1000:1 to 10:1. The polymers and copolymers can be producedby first preparing a polyamic acid intermediate in situ. The polyamicacid intermediate can then be transformed into a polyamic amide polymer,a polyimide polymer, a polyisoimide polymer, or a mixture thereof. Inone embodiment, the polyamic acid can be further reacted with a nitrogencontaining hydrocarbon to form a polyamic amide polymer. The polymer canthen be thermally treated to reduce the amount of polyisoimide and/orpolyamic amide and form increased amounts of polyimide polymer.

In another embodiment, polyamic acid can be formed into a sheet or afilm and subsequently processed with heat (often temperatures higherthan 250° C.) or both heat and catalysts to convert the polyamic acid toa polyimide. This process can also be applied after the polyamic acidhas been treated with a nitrogen containing hydrocarbon to prepare amixed polymer containing both polyamic amide monomer and polyimidemonomer or copolymer. A copolymer can also be prepared by controllingthe amount of amidation versus imidization. One method to control theratio of m:n includes limiting or providing in excess the nitrogencontaining hydrocarbon, such as a secondary amine in the reactionavailable for amidation. Without being limited to theory, it is believedthat controlling the amount, or type of, dehydration agent, temperature,solvent, and/or reaction time can also contribute to the ratio of m:n.Another benefit of an aerogel polymer composition containing polyamicamide polymers is that during the formation of the polyamic amide littleto no polyisoimide is formed.

In some instances, the polyamic acid intermediate can be moisturesensitive, and care must be taken to avoid the uptake of water into thepolymer solution. Additionally, some polyamic acid intermediates exhibitself-imidization in solution as they gradually convert to the polyimidestructure. The imidization reaction can reduce the polymer solubilityand produce water as a by-product. The produced water can then reactwith the remaining polyamic acid, thereby cleaving the polymer chain,thus polyamic acids are used, in some instances, in situ, or directlyafter isolation.

In some aspects, the precursors or intermediates that are formed to makethe aerogel polymer composition, including polyamic acid, polyamic acidsalt precursors, or polyamic amide precursors can be soluble in thereaction solvent. In this instance, the soluble precursor solutions canbe cast into a film on a suitable substrate such as by spin casting,gravure coating, three roll coating, knife over roll coating, slot dieextrusion, dip coating, Meyer rod coating, or other techniques. The castfilm can then be heated in stages to elevated temperatures to removesolvent and convert, for example, the amic acid functional groups in theprecursor to polyamic amide through amidation with an appropriatenitrogen containing hydrocarbon, to polyimide by imidization, or byapplying appropriate conditions to afford a mixed copolymer.

One class of monomer used to prepare the polymers and copolymers of thecurrent invention can be a diamine, or a diamine monomer. The diaminemonomer can also be a diisocyanate, and it is to be understood that anisocyanate could be substituted for an amine in this description, asappropriate. The other type of monomer can be an acid monomer, (e.g., adianhydride) or a di-acid monomer. Di-acid monomers can include adianhydride, a tetraester, a diester acid, a tetracarboxylic acid, or atrimethylsilyl ester, all of which can react with a diamine to produce apolyamic acid intermediate that can be used to prepare a polyamic amidepolymer or copolymer. Dianhydrides are sometimes referred to in thisdescription, but it is to be understood that tetraesters, diester acids,tetracarboxylic acids, or trimethylsilyl esters could be substituted, asappropriate.

Because one di-acid monomer has two anhydride groups, different diaminomonomers can react with each anhydride group so the di-acid monomer canbecome located between two different diamino monomers. The diaminemonomer contains two amine functional groups; therefore, after the firstamine functional group attaches to one di-acid monomer, the second aminefunctional group is still available to attach to another di-acidmonomer, which then attaches to another diamine monomer, and so on. Inthis manner, the polymer backbone is formed. The resultingpolycondensation reaction forms a polyamic acid.

The aerogel polymer compositions containing polyamic amide polymer areusually formed from two different types of monomers, and it is possibleto mix different varieties of each type of monomer. Therefore, one, two,or more di-acid monomers can be included in the reaction vessel, as wellas one, two or more diamino monomers. The total molar quantity ofdi-acid monomers is kept about the same as the total molar quantity ofdiamino monomers if a long polymer chain is desired. Because more thanone type of diamine or di-acid can be used, the various monomerconstituents of each polymer chain can be varied to produce aerogelpolymer compositions with different properties. For example, a singlediamine monomer AA can be reacted with two di-acid co monomers, B₁B₁ andB₂B₂, to form a polymer chain of the general form(AA-B₁B₁)_(x)-(AA-B₂B₂)_(y) in which x and y are determined by therelative incorporations of B₁B₁ and B₂B₂ into the polymer backbone.Alternatively, diamine co-monomers A₁A₁ and A₂A₂ can be reacted with asingle di-acid monomer BB to form a polymer chain of the general form of(A₁A₁-BB)_(x)-(A₂A₂-BB)_(y). Additionally, two diamine co-monomers A₁A₁and A₂A₂ can be reacted with two di-acid co-monomers B₁B₁ and B₂B₂ toform a polymer chain of the general form(A₁A₁-B₁B₁)_(w)-(A₁A₁-B₂B₂)_(x)-(A₂A₂-B₁B₁)_(y)-(A₂A₂-B₂B₂)_(z), wherew, x, y, and z are determined by the relative incorporation of A₁A₁-B₁,A₁A₁-B₂B₂, A₂A₂-B₁B₁, and A₂A₂-B₂B₂ into the polymer backbone. More thantwo di-acid co-monomers and/or more than two diamine co-monomers canalso be used. Therefore, one or more diamine monomers can be polymerizedwith one or more di-acids, and the general form of the polymer isdetermined by varying the amount and types of monomers used.

There are many examples of monomers that can be used to make the aerogelpolymer compositions containing polyamic amide polymer of the presentinvention. In some embodiments, the diamine monomer is a substituted orunsubstituted aromatic diamine, a substituted or unsubstitutedalkyldiamine, or a diamine that can include both aromatic and alkylfunctional groups. A non-limiting list of possible diamine monomersinclude 4,4′-oxydianiline, 3,4′-oxydianiline, 3,3′-oxydianiline,p-phenylenediamine, m-phenylenediamine, o-phenylenediamine,diaminobenzanilide, 3,5-diaminobenzoic acid,3,3′-diaminodiphenylsulfone, 4,4′-diaminodiphenylsulfones,1,3-bis-(4-aminophenoxy)benzene, 1,3-bis-(3-aminophenoxy)benzene,1,4-bis-(4-aminophenoxy)benzene, 1,4-bis-(3-aminophenoxy)benzene,2,2-Bis[4-(4-aminophenoxy)phenyl]-hexafluoropropane,2,2-bis(3-aminophenyl)-1,1,1,3,3,3-hexafluoropropane,4,4′-isopropylidenedianiline,1-(4-aminophenoxy)-3-(3-aminophenoxy)benzene,1-(4-aminophenoxy)-4-(3-aminophenoxy)benzene,bis[4-(4-aminophenoxy)phenyl]sulfones,2,2-bis[4-(3-aminophenoxy)phenyl]sulfones,bis(4-[4-aminophenoxy]phenyl)ether,2,2′-bis-(4-aminophenyl)-hexafluoropropane, (6F-diamine),2,2′-bis-(4-phenoxyaniline)isopropylidene, meta-phenylenediamine,para-phenylenediamine, 1,2-diaminobenzene, 4,4′-diaminodiphenylmethane,2,2-bis(4-aminophenyl)propane, 4,4′diaminodiphenylpropane,4,4′-diaminodiphenylsulfide, 4,4′-diaminodiphenylsulfone,3,4′diaminodiphenylether, 4,4′-diaminodiphenylether,2,6-diaminopyridine, bis(3-aminophenyl)diethyl silane,4,4′-diaminodiphenyl diethyl silane, benzidine, dichlorobenzidine,3,3′-dimethoxybenzidine, 4,4′-diaminobenzophenone,N,N-bis(4-aminophenyl)-n-butylamine, N,N-bis(4-aminophenyl)methylamine,1,5-diaminonaphthalene, 3,3′-dimethyl-4,4′-diaminobiphenyl,4-aminophenyl-3-aminobenzoate, N,N-bis(4-aminophenyl)aniline,bis(p-beta-amino-t-butylphenyl)ether,p-bis-2-(2-methyl-4-aminopentyl)benzene,p-bis(1,1-dimethyl-5-aminopentyl)benzene,1,3-bis(4-aminophenoxy)benzene, m-xylenediamine, p-xylenediamine,4,4′-diaminodiphenyletherphosphine oxide, 4,4′-diaminodiphenylN-methylamine, 4,4′-diaminodiphenyl N-phenylamine, amino-terminalpolydimethylsiloxanes, amino-terminal polypropyleneoxides,amino-terminal polybutyleneoxides,4,4′-methylenebis(2-methylcyclohexylamine), 1,2-diaminoethane,1,3-diaminopropane, 1,4-diaminobutane, 1,5-diaminopentane,1,6-diaminohexane, 1,7-diaminoheptane, 1,8-diaminooctane,1,9-diaminononane, 1,10-diaminodecane, 4,4′-methylenebisbenzeneamine,2,2′-dimethylbenzidine, (also known as4,4′-diamino-2,2′-dimethylbiphenyl (DMB), bisaniline-p-xylidene,4,4′-bis(4-aminophenoxy)biphenyl, 3,3′-bis(4 aminophenoxy)biphenyl,4,4′-(1,4-phenylenediisopropylidene)bisaniline, and4,4′-(1,3-phenylenediisopropylidene)bisaniline, or combinations thereof.In a specified embodiment, the diamine monomer is 4,4′-oxydianiline,2,2′-dimethylbenzidine, or both.

A non-limiting list of possible dianhydride monomers includehydroquinone dianhydride, 3,3,4,4′-biphenyltetracarboxylic dianhydride(BPDA), pyromellitic dianhydride, 3,3′,4,4′-benzophenonetetracarboxylicdianhydride, 4,4′-oxydiphthalic anhydride,3,3′,4,4′-diphenylsulfonetetracarboxylic dianhydride,4,4′-(4,4′-isopropylidenediphenoxy)bis(phthalic anhydride),2,2-bis(3,4-dicarboxyphenyl)propane dianhydride,4,4′-(hexafluoroisopropylidene)diphthalic anhydride,bis(3,4-dicarboxyphenyl)sulfoxide dianhydride, polysiloxane-containingdianhydride, 2,2′,3,3′-biphenyltetracarboxylic dianhydride,2,3,2′,3′-benzophenonetetraearboxylic dianhydride,3,3′,4,4′-benzophenonetetraearboxylic dianhydride,naphthalene-2,3,6,7-tetracarboxylic dianhydride,naphthalene-1,4,5,8-tetracarboxylie dianhydride, 4,4′-oxydiphthalicdianhydride, 3,3′,4,4′-biphenylsulfonetetracarboxylic dianhydride,3,4,9,10-perylene tetracarboxylic dianhydride,bis(3,4-dicarboxyphenyl)sulfide dianhydride,bis(3,4-dicarboxyphenyl)methane dianhydride,2,2-bis(3,4-dicarboxyphenyl)propane dianhydride,2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane,2,6-dichloronaphthalene-1,4,5,8-tetracarboxylic dianhydride,2,7-dichloronapthalene-1,4,5,8-tetracarboxylic dianhydride,2,3,6,7-tetrachloronaphthalene-1,4,5 ,8-tetracarboxylic dianhydride,phenanthrene-, 8,9,10-tetracarboxylie dianhydride,pyrazine-2,3,5,6-tetracarboxylic dianhydride,benzene-1,2,3,4-tetracarboxylic dianhydride, andthiophene-2,3,4,5-tetracarboxylic dianhydride, or combinations thereof.In a specific embodiment, the dianhydride monomer is3,3′,4,4′-biphenyltetracarboxylic dianhydride, pyromellitic dianhydride,or both.

In some aspects, the molar ratio of anhydride to total diamine is from0.4:1 to 1.6:1, 0.5:1 to 1.5:1, 0.6:1 to 1.4:1, 0.7:1 to 1.3:1, orspecifically from 0.8:1 to 1.2:1. In further aspects, the molar ratio ofdianhydride to multifunctional amine (e.g., triamine) is 2:1 to 140:1,3:1 to 130:1, 4:1 to 120:1, 5:1 to 110:1, 6:1 to 100:1, 7:1 to 90:1, orspecifically from 8:1 to 80:1. The polymer can also include amono-anhydride group, including for example 4-amino-1,8-naphthalicanhydride, endo-bicyclo[2.2.2]oct-5-ene-2,3-dicarboxylic anhydride,citraconic anhydride, trans-1,2-cyclohexanedicarboxylic anhydride,3,6-dichlorophthalic anhydride, 4,5-dichlorophthalic anhydride,tetrachlorophthalic anhydride 3,6-difluorophthalic anhydride,4,5-difluorophthalic anhydride, tetrafluorophthalic anhydride, maleicanhydride, 1-cyclopentene-1,2-dicarboxylic anhydride,2,2-dimethylglutaric anhydride 3,3-dimethylglutaric anhydride,2,3-dimethylmaleic anhydride, 2,2-dimethylsuccinic anhydride,2,3-diphenylmaleic anhydride, phthalic anhydride, 3-methylglutaricanhydride, methylsuccinic anhydride, 3-nitrophthalic anhydride,4-nitrophthalic anhydride, 2,3-pyrazinedicarboxylic anhydride, or3,4-pyridinedicarboxylic anhydride. Specifically, the mono-anhydridegroup is phthalic anhydride.

In another embodiment, the polymer compositions used to prepare theaerogels of the present invention include multifunctional amine monomerswith at least three primary amine functionalities. The multifunctionalamine may be a substituted or unsubstituted aliphatic multifunctionalamine, a substituted or unsubstituted aromatic multifunctional amine, ora multifunctional amine that includes a combination of an aliphatic andtwo aromatic groups, or a combination of an aromatic and two aliphaticgroups. A non-limiting list of possible multifunctional amines includepropane-1,2,3-triamine, 2-aminomethylpropane-1,3-diamine,3-(2-aminoethyl)pentane-1,5-diamine, bis(hexamethylene)triamine,N′,N′-bis(2-aminoethyl)ethane-1,2-diamine,N′,N′-bis(3-aminopropyl)propane-1,3-diamine,4-(3-aminopropyl)heptane-1,7-diamine,N′,N′-bis(6-aminohexyl)hexane-1,6-diamine, benzene-1,3,5-triamine,cyclohexane-1,3,5-triamine, melamine,N-2-dimethyl-1,2,3-propanetriamine, diethylenetriamine, 1-methyl or1-ethyl or 1-propyl or 1-benzyl-substituted diethylenetriamine,1,2-dibenzyldiethylenetriamine, lauryldiethylenetriamine,N-(2-hydroxypropyl)diethylenetriamine,N,N-bis(1-methylheptyl)-N-2-dimethyl-1,2,3-propanetriamine,2,4,6-tris(4-(4-aminophenoxy)phenyl)pyridine,N,N-dibutyl-N-2-dimethyl-1,2,3 -propanetriamine,4,4′-(2-(4-aminobenzyl)propane-1,3-diyl)dianiline,4-((bis(4-aminobenzyl)amino)methyl)aniline,4-(2-(bis(4-aminophenethyl)amino)ethyl)aniline,4,4′-(3-(4-aminophenethyl)pentane-1,5-diyl)dianiline,1,3,5-tris(4-aminophenoxy)benzene (TAPOB),4,4′,4″-methanetriyltrianiline,N,N,N′,N′-Tetrakis(4-aminophenyl)-1,4-phenylenediamine, apolyoxypropylenetriamine, octa(aminophenyl)polyhedral oligomericsilsesquioxane, or combinations thereof. A specific example of apolyoxypropylenetriamine is JEFFAMINE® T-403 from Huntsman Corporation,The Woodlands, Tex. USA. In a specific embodiment, the aromaticmultifunctional amine may be 1,3,5-tris(4-aminophenoxy)benzene or4,4′,4″-methanetriyltrianiline. In some embodiments, the multifunctionalamine includes three primary amine groups and one or more secondaryand/or tertiary amine groups, for example,N′,N′-bis(4-aminophenyl)benzene-1,4-diamine.

Non-limiting examples of capping agents or groups include amines,maleimides, nadimides, acetylene, biphenylenes, norbornenes,cycloalkyls, and N-propargyl and specifically those derived fromreagents including 5-norbornene-2,3-dicarboxylic anhydride (nadicanhydride, NA), methyl-nadic anhydride, hexachloro-nadic anhydride,cis-4-cyclohexene-1,2-dicarboxylic anhydride,4-amino-N-propargylphthalimide, 4-ethynylphthalic anhydride, and maleicanhydride.

In some instances, the backbone of the aerogel polymer compositions caninclude further substituents. The substituents (e.g., oligomers,functional groups, etc.) can be directly bonded to the backbone orlinked to the backbone through a linking group (e.g., a tether or aflexible tether). In other embodiments, a compound or particles can beincorporated (e.g., blended and/or encapsulated) into the polymerstructure without being covalently bound to the polymer structure.

In some instances, the incorporation of the compound or particles can beperformed during the any step of the reaction process. In someinstances, particles can aggregate, thereby producing polyamic amide orpolyimide having domains with different concentrations of thenon-covalently bound compounds or particles.

In one instance, an aerogels of the present invention can include lessthan 5 wt. % of the polyamic amide polymer based on the total weight ofpolymer aerogel. In one particular instance, an aerogel of the presentinvention can include 0.01 wt. % to 4.95 wt. % of the polyamic amidepolymer based on the formula weight of the tertiary amine (e.g.,2-methylimidazole). In another instance, an aerogel of the presentinvention can include 0.01 wt. % to 1 wt. % of the polyamic amidepolymer based on the formula weight of the tertiary amine (e.g.,2-methylimidazole). An aerogel of the present invention can includegreater than any one of, equal to any one of, or between any two of0.01, 0.05, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6,0.65, 0.7, 0.75, 0.8, 0.85, 0.9, 0.95, 1.0, 1.5, 2, 2.5, 3, 3.5, 4, 4.5,4.6, 4.7, 4.8, and 4.95 wt. % of the polyamic amide polymer based on thetotal weight of the polymer aerogel.

E. Preparation of Aerogels

Aerogels of the present disclosure can be made using a multi-stepprocess that includes 1) preparation of the polyamic amide gel, 2)optional solvent exchange, 3) drying of the polymeric solution to formthe aerogel and 4) heat-treating the formed aerogel to reduce the amountof non-polyimide species. These process steps are discussed in moredetail below.

FIG. 1 is a non-limiting reaction schematic showing a conventionalsynthesis of a polyimide polymer that generate a polyisoimide byproduct.Diamine 100 can be mixed with dianhydride 102 under reaction conditions104 to form polyamic acid intermediate 106 that is further treated witha tertiary amine and dehydration agent under reaction conditions 108 toform polyimide 110 and polyisoimide 112. In contrast to the conventionalsynthesis, the method of the present invention produces a copolymerhaving a polyimide repeating unit and a polyamic amide repeating unit.FIG. 2 is a non-limiting reaction schematic showing in anotherembodiment the synthesis of a polyimide polymer including polyamic amideinstead of polyisoimide. Polyamic acid intermediate 106 can be treatedwith a nitrogen containing hydrocarbon containing a secondary andtertiary amine and a dehydration agent under reaction conditions 200 toform polyimide 110 and polyamic amide 202.

1. Polyamic Amide Gels

The method to prepare a polyamic amide can include (a) providing atleast one diamine compound to a solvent to form a solution; (b)providing at least one dianhydride compound to the solution of step (a)under conditions sufficient to form a polyamic acid solution; (c)providing a secondary amine to the polyamic acid solution; (d)subjecting the mixture of step (c) to conditions suitable to produce apolymer matrix solution including a polyamic amide; and (e) subjectingthe polymer matrix solution to conditions sufficient to form an aerogel.As discussed above, numerous acid monomers, diamino monomers, andmultifunctional amine monomers can be used to synthesize a polyamicamide having minimal or no cross-linking. In one aspect of the currentinvention, one or more diamino monomers and one or more multifunctionalamine monomers are premixed in one or more solvents and then treatedwith one or more dianhydrides (e.g., di-acid monomers) that are added insequentially smaller amounts at pre-defined time increments whilemonitoring the viscosity. The desired viscosity of the polymerizedsolution can range from 50 to 20,000 cP or specifically 500 to 5,000 cP.By performing the reaction using incremental addition of dianhydridewhile monitoring viscosity, a non-crosslinked aerogel can be prepared.For instance, a triamine monomer (23 equiv.) can be added to the solventto give a 0.0081 molar solution. To the solution a first diamine monomer(280 equiv.) can be added, followed by second diamine monomer (280equiv.). Next a dianhydride (552 total equiv.) can be added insequentially smaller amounts at pre-defined time increments whilemonitoring the viscosity. The dianhydride can be added until theviscosity reaches 1,000 to 1,500 cP. For example, a first portion ofdianhydride can be added, the reaction can be stirred (e.g., for 20minutes), a second portion of dianhydride can be added, and a sample ofthe reaction mixture was then analyzed for viscosity. After stirring foradditional time (e.g., for 20 minutes), a third portion of dianhydridecan be added, and a sample can be taken for analysis of viscosity. Afterfurther stirring for a desired period of time (e.g., 10 hours to 12hours), a mono-anhydride (96 equiv.) can be added. After having reachedthe target viscosity, the reaction mixture can be stirred for a desiredperiod of time (e.g., 10 hours to 12 hours) or the reaction is deemedcompleted. The reaction temperature can be determined by routineexperimentation depending on the starting materials. In a preferredembodiment, the temperature range can be greater than any one of, equalto any one of, or between any two of 20° C., 30° C., 35° C., 40° C., and45° C. After a desired amount of time (e.g., about 2 hours), the productcan be isolated (e.g., filtered), after which a nitrogen containinghydrocarbon (828 equiv.) and dehydration agent (1214 equiv.) can beadded. The addition of the nitrogen containing hydrocarbon and/ordehydration agent can occur at any temperature. In some embodiments, thenitrogen containing hydrocarbon and/or dehydration agent is added to thesolution at 20° C. to 28° C. (e.g., room temperature) stirred for adesired amount of time at room temperature. In some instances, afteraddition of nitrogen containing hydrocarbon and/or dehydration agent,the solution temperature is raised up to 150° C.

The reaction solvent can be dimethylsulfoxide, diethylsulfoxide,N,N-dimethylformamide, N,N-diethylformamide, N,N-dimethylacetamide,N,N-diethylacetamide, N-methyl-2-pyrrolidone, 1-methyl-2-pyrrolidinone,N-cyclohexyl-2-pyrrolidone, 1,13 -dimethyl-2-imidazolidinone,diethyleneglycoldimethoxyether, o-dichlorobenzene, phenols, cresols,xylenol, catechol, butyrolactones, hexamethylphosphoramide, or mixturesthereof. The reaction solvent and other reactants can be selected basedon the compatibility with the materials and methods applied i.e. if thepolymerized polyamic amide gel is to be cast onto a support film,injected into a moldable part, or poured into a shape for furtherprocessing into a workpiece. In a specific embodiment, the reactionsolvent is dimethylsulfoxide.

In some aspects, a chemical curing system suitable for driving theconversion of polymer precursor to the polyamic amide or polyimide statecan be employed. Chemical catalysts can include nitrogen containinghydrocarbons. Non-limiting examples of such compounds include compoundscontaining at least one secondary amine. In one particular instance, anorganic compound having a secondary and a tertiary amine, such as2-methylimidazole or 2-ethyl-4-methylimidazole can be used as a chemicalcatalyst. In some embodiments, the secondary amines can be used incombination with other chemical catalysts such as pyridine,methylpyridines, quinoline, isoquinoline,1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), DBU phenol salts, carboxylicacid salts of DBU, triethylenediamine, carboxylic acid slats oftriethylenediamine, lutidine, N-methylmorpholine, triethylamine,tripropylamine, tributylamine, other trialkylamines, or combinationsthereof. Any dehydrating agent suitable for amidation can be used in themethods of the present invention. Dehydrating agents may include aceticanhydride, propionic anhydride, n-butyric anhydride, benzoic anhydride,trifluoroacetic anhydride, oxalyl chloride, thionyl chloride, phosphorustrichloride, dicyclohexylcarbodiimide, 1,1′-carbonyldiimidazole (CDI),di-tert-butyl dicarbonate (Boc₂O), or combinations thereof. Amidationcan also be achieved by using standard peptide coupling reagents such asbenzotriazol-1-yl-oxytripyrrolidinophosphonium hexafluorophosphate(PyBOP) or1-[Bis(dimethylamino)methylene]-1H-1,2,3-triazolo[4,5-b]pyridinium3-oxid hexafluorophosphate (HATU) in the presence of a base such asN,N-diisopropylethylamine (DIPEA), and a solvent, such as DMF and thelike.

While keeping the above in mind, the introduction of macropores into theaerogel polymeric matrix, as well as modifying or tuning the amount ofsuch macropores present, can be performed in the manner described abovein the Summary of the Invention Section as well as throughout thisspecification. In one non-limiting manner, the formation of macroporesvs smaller mesopores and micropores can be primarily controlled bycontrolling the polymer/solvent dynamics during gel formation. By doingso, the pore structure can be controlled, and the quantity and volume ofmacroporous, mesoporous, microporous cells can be controlled. Forexample, a curing additive that reduces the resultant polyimidesolubility, such as 1,4-diazabicyclo[2.2.2]octane, produces a polyimidecontaining a higher number of macropores compared to another curingadditive that improves the resultant polymer solubility, such astrimethylamine. In another example, using the same dianhydride such asBPDA but increasing the ratio of rigid amines incorporated into thepolymer backbone such as p-PDA as compared to more flexible diaminessuch as 4,4′-ODA, the formation of macropores as compared to smallermesopores and micropores can be controlled.

In some embodiments, the polyamic amide solution can be cast onto acasting sheet covered by a support film for a period of time. In certainembodiments, the casting sheet is a polyethylene terephthalate (PET)casting sheet. After a passage of time, the polymerized gel can beremoved from the casting sheet and prepared for the solvent exchangeprocess.

2. Solvent Exchange

After the polyamic amide gel is synthesized, it can be subjected to asolvent exchange where the reaction solvent is exchanged for a moredesirable second solvent. The original solvent can be exchanged with asecond solvent having a higher volatility than the first solvent andrepeated with various solvents. By way of example, the polymerized gelcan be placed inside of a pressure vessel and submerged in a mixturethat includes the reaction solvent and the second solvent. Then, a highpressure atmosphere can be created inside of the pressure vessel therebyforcing the second solvent into the polymerized gel and displacing aportion of the reaction solvent. Alternatively, the solvent exchangestep can be conducted without the use of a high pressure environment. Itmay be necessary to conduct a plurality of rounds of solvent exchange.

The time necessary to conduct the solvent exchange can vary dependingupon the type of polymer undergoing the exchange as well as the reactionsolvent and second solvent being used. In one embodiment, each solventexchange can range from 1 to 168 hours or any period time there betweenincluding 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18,19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36,37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54,55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72,73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90,91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104, 105, 106,107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120,121,1 22, 123, 124, 125, 126, 127, 128, 129, 130, 131, 132, 133, 134,135, 136, 137, 138, 139, 140, 141, 142, 143, 144, 145, 146, 147, 148,149, 150, 151, 152, 153, 154, 155, 156, 157, 158, 159, 160, 161, 162,163, 164, 165, 166, or 167 hours. In another embodiment, each solventexchange can take approximately 30 minutes. Exemplary second solventsinclude methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol,isobutanol, tert-butanol, 3-methyl-2-butanol, 3,3-dimethyl-2-butanol,2-pentanol, 3-pentanol, 2,2-dimethylpropan-1-ol, cyclohexanol,diethylene glycol, cyclohexanone, acetone, acetyl acetone, 1,4-dioxane,diethyl ether, dichloromethane, trichloroethylene, chloroform, carbontetrachloride, water, and mixtures thereof. In a specific embodiment,the second solvent is acetone. In certain non-limiting embodiments, thesecond solvent can have a suitable freezing point for performingsupercritical or subcritical drying steps. For example tert-butylalcohol has a freezing point of 25.5° C. and water has a freezing pointof 0° C. under one atmosphere of pressure. Alternatively, and asdiscussed below, however, the drying can be performed without the use ofsupercritical or subcritical drying steps, such as by evaporative dryingtechniques.

The temperature and pressure used in the solvent exchange process can bevaried. The duration of the solvent exchange process can be adjusted byperforming the solvent exchange at a varying temperatures or atmosphericpressures, or both, provided that the pressure and temperature insidethe pressure vessel does not cause either the first solvent or thesecond solvent to leave the liquid phase and become gaseous phase, vaporphase, solid phase, or supercritical fluid. Generally, higher pressuresand/or temperatures decrease the amount of time required to perform thesolvent exchange, and lower temperatures and/or pressures increase theamount of time required to perform the solvent exchange.

3. Cooling and Drying

In some embodiments after solvent exchange, the polymerized gel can bedried. The drying step can include supercritical drying, subcriticaldrying, thermal drying, evaporative air-drying, or any combinationthereof. In some embodiments, the polymerized gel can be exposed tosupercritical drying. In this instance the solvent in the gel can beremoved by supercritical CO₂ extraction.

In another embodiment after solvent exchange, the polymerized gel can beexposed to subcritical drying. In this instance, the gel can be cooledbelow the freezing point of the second solvent and subjected to a freezedrying or lyophilization process to produce the aerogel. For example, ifthe second solvent is water, then the polymerized gel is cooled to below0° C. After cooling, the polymerized gel is subjected to a vacuum for aperiod of time wherein the second solvent is allowed to sublime.

In still another embodiment after solvent exchange, the polymerized gelcan be exposed to subcritical drying with optional heating after themajority of the second solvent has been removed through sublimation. Inthis instance, the partially dried gel material is heated to atemperature near, or above, the boiling point of the second solvent fora period of time. The period of time can range from a few hours toseveral days, although a typical period of time is approximately 4hours. During the sublimation process, a portion of the second solventpresent in the polymerized gel has been removed, leaving a gel that canhave macropores, mesopores, or micropores, or any combination thereof orall of such pore sizes. After the sublimation process is complete, ornearly complete, the aerogel has been formed.

In yet another embodiment after solvent exchange, the polymerized gelcan be dried under ambient conditions, for example, by removing thesolvent under a stream of gas (e.g., air, anhydrous gas, inert gas(e.g., nitrogen (N₂) gas), etc. Still further, passive drying techniquescan be used such as simply exposing the gel to ambient conditionswithout the use of a gaseous stream. In this instance, the solvent inthe gel is removed by evaporation and pore collapse is prevented by theaerogel network. The drying may also be assisted by heating orirradiating with electromagnetic radiation.

4. Thermal Treatment

The dried aerogel can be thermally treated at 275° C. to 550° C., 285°C. to 450° C. or greater than any one of, equal to any one of, orbetween any two of 275° C., 280° C., 285° C., 290° C., 295° C., 300° C.,305° C., 310° C., 350° C., 400° C., 450° C., 500° C., and 550° C. for adesired amount of time (e.g., 5 to 12 hours) to produce a thermallytreated polyimide aerogel that contains less polyamic amide and/orpolyisoimide than prior to thermal treatment. The heating can beperformed under an inert atmosphere (e.g., nitrogen, argon, or heliumatmosphere), in a vacuum, or in air. Thermally treating under theseconditions can remove any material not chemically bound to the polymermatrix (e.g., 2-methylimidazole). By way of example, FIG. 3 depicts areaction schematic illustrating polyamic acid and polyamic amidepolymers of the present invention that can be converted to a polyamicimide polymer. In some embodiments, the amount of polyamic amide can bereduced by at least 15%, 20%, 25%, 30%, 40%, 50%, 60%, 70%, 80%, 90% or99%. In some embodiments, the aerogel is heated until a weight change ofless than 1 wt. % is observed, off-gassing is not detected, or both.Off-gassing can be determined using ASTM E595-15. In some embodiments,the aerogel can be thermally treated until a total mass loss is lessthan 2 wt. %, 1.5 wt. %, 1.0 wt. % 0.5 wt. % and/or the collectedvolatile condensable materials are less than 0.5 wt. %, 0.4 wt. %, 0.3wt. %, 0.2 wt. %, 0.1 wt. % or 0.0 wt. %.

In some embodiments, the thermally treated aerogel can be subjected to asecond or more thermal treatments. Further heat treatment (e.g., drying)can remove any compounds not chemically bound to the polymer matrix(e.g., water, residual solvents, 2-methylimidazole, or benzoic acid). Byway of example, the thermally treated aerogel can be heated at atemperature greater than any one of, equal to any one of, or between anytwo of 225° C., 250° C., 300° C. to 310° C. under vacuum conditionsunder inert conditions (e.g., under an inert gas) or in air. In someembodiments, the thermally treated aerogel can be subjected to anadditional thermal treatment at a temperature of greater than any oneof, equal to any one of, or between any two of 225° C., 250° C., 300° C.to 310° C. under a gas flow (e.g., an air flow, inert gas flow, etc.).In some embodiments, the thermally treated aerogel is cooled to belowthe second heating temperature (e.g., cooled to less than 225° C., 200°C., 150° C., 100° C., 50° C. or 25° C.) and then heated.

F. Articles of Manufacture

The open-cell aerogel of the present invention can be included in anarticle of manufacture. For example, an article of manufacture caninclude a branched thermally treated polyamic amide aerogel matrix withless than 5% by weight of crosslinked polymers. In some embodiments, thearticle of manufacture is a thin film, monolith, wafer, blanket, corecomposite material, substrate for radiofrequency antenna, a sunscreen, asunshield, a radome, insulating material for oil and/or gas pipeline,insulating material for liquefied natural gas pipeline, insulatingmaterial for cryogenic fluid transfer pipeline, insulating material forapparel, insulating material for aerospace applications, insulatingmaterial for buildings, cars, and other human habitats, insulatingmaterial for automotive applications, insulation for radiators,insulation for ducting and ventilation, insulation for air conditioning,insulation for heating and refrigeration and mobile air conditioningunits, insulation for coolers, insulation for packaging, insulation forconsumer goods, vibration dampening, wire and cable insulation,insulation for medical devices, support for catalysts, support fordrugs, pharmaceuticals, and/or drug delivery systems, aqueous filtrationapparatus, oil-based filtration apparatus, and solvent-based filtrationapparatus.

1. Fluid Filtration Applications

In some embodiments, the open-cell aerogel of the present invention canbe used in fluid filtration systems and apparatus. A feed fluid can becontacted with the branched thermally treated polyamic amide aerogelsuch that some, all or, substantially all, of the impurities and/ordesired substances are removed from the feed fluid to produce a filtrateessentially devoid of the impurities and/or desired substances. Thefiltrate, impurities, and/or desired substances can be collected,stored, transported, recycled, or further processed. The thermallytreated aerogel can be further processed to release the impuritiesand/or desired substances from the aerogel.

The thermally treated polyamic amide aerogel described herein can beused in or with filtration apparatuses known in the art. Non-limitingexamples of filtration apparatuses and applications include gas filterssuch as, but not limited to, building air filters, automotive cabin airfilters, combustion engine air filters, aircraft air filters, satelliteair filters, face mask filters, diesel particulate filters, in-line gasfilters, cylinder gas filters, soot filters, pressure swing absorptionapparatus, etc. Additional non-limiting examples of filtrationapparatuses and applications include solvent filtration systems, columnfiltration, chromatography filtration, vacuum flask filtration,microfiltration, ultrafiltration, reverse osmosis filtration,nanofiltration, centrifugal filtration, gravity filtration, cross flowfiltration, dialysis, hemofiltration, hydraulic oil filtration,automotive oil filtration, etc. Further, non-limiting examples of thepurpose of filtration includes sterilization, separation, purification,isolation, etc.

A fluid for filtration (“feed”) and a filtrate can be any fluid. Thefluid can be a liquid, gas, supercritical fluid, emulsion, or mixturethereof. In some instances, the liquid can be aqueous, non-aqueous,organic, non-organic, biological in origin, or a mixture thereof. Insome instances, the gas can include air, nitrogen, oxygen, an inert gas,or mixtures thereof. In some instances, the liquid can contain solidsand/or other fluids or be an emulsion. In particular instances theemulsion is a water-oil emulsion, an oil-water emulsion, a water-solventemulsion, a solvent-water emulsion, an oil-solvent emulsion, or asolvent-oil emulsion. As non-limiting examples, the liquid can be water,blood, plasma, an oil, a solvent, air, or mixtures thereof. The solventcan be an organic solvent. Water can include water, any form of steamand supercritical water.

In some instances, the fluid can contain impurities. Non-limitingexamples of impurities include solids, liquids, gases, supercriticalfluids, objects, compounds, and/or chemicals, etc. What is defined as animpurity may be different for the same feed fluid depending on thefiltrate desired. In some embodiments, one or more aerogels can be usedto remove impurities. Non-limiting examples of impurities in water caninclude ionic substances such as sodium, potassium, magnesium, calcium,fluoride, chloride, bromide, sulfate, sulfite, nitrate, nitrites,cationic surfactants, and anionic surfactants, metals, heavy metals,suspended, partially dissolved, or dissolved oils, organic solvents,nonionic surfactants, defoamants, chelating agents, microorganisms,particulate matter, etc. Non-limiting examples of impurities in bloodcan include red blood cells, white blood cells, antibodies,microorganisms, water, urea, potassium, phosphorus, gases, particulatematter, etc. Non-limiting examples of impurities in oil can includewater, particulate matter, heavy and/or light weight hydrocarbons,metals, sulfur, defoamants, etc. Non-limiting examples of impurities insolvents can include water, particulate matter, metals, gases, etc.Non-limiting impurities in air can include water, particulate matter,microorganisms, liquids, carbon monoxide, sulfur dioxide, etc.

In some instances, the feed fluid can contain desired substances.Desired substances can be, but are not limited to, solids, liquids,gases, supercritical fluids, objects, compounds, and/or chemicals, etc.In some embodiments, one or more aerogels can be used to concentrate orcapture a desired substance, or remove a fluid from a desired substance.Non-limiting examples of desired substances in water can include ionicsubstances such as sodium, potassium, magnesium, calcium, fluoride,chloride, bromide, sulfate, sulfite, nitrate, nitrites, cationicsurfactants, and anionic surfactants, metals, heavy metals, suspended,partially dissolved, or dissolved oils, organic solvents, nonionicsurfactants, chelating agents, microorganisms, particulate matter, etc.Non-limiting examples of desired substances in blood can include redblood cells, white blood cells, antibodies, lipids, proteins, etc.Non-limiting examples of desired substances in oil can includehydrocarbons of a range of molecular weights, gases, metals, defoamants,etc. Non-limiting examples of desired substances in solvents can includeparticulate matter, fluids, gases, proteins, lipids, etc. Non-limitingexamples of desired substances in air can include water, fluids, gases,particulate matter, etc.

The compatibility of an aerogel with a fluid and/or filtrationapplication can be determined by methods known in the art. Someproperties of an aerogel that may be determined to assess thecompatibility of the aerogel may include, but is not limited to: thetemperature and/or pressures that the aerogel melts, dissolves,oxidizes, reacts, degrades, or breaks; the solubility of the aerogel inthe material that will contact the aerogel; the flow rate of the fluidthrough the aerogel; the retention rate of the impurity and/or desiredproduct form the feed fluid; etc.

2. Radiofrequency (RF) Applications

Due to their low density, mechanical robustness, light weight, and lowdielectric properties, the branched thermally treated polyamic amideaerogel aerogels can be used in radiofrequency (RF) applications. Theuse of thermally treated aerogels of the present invention in RFapplications enables the design of thinner substrates, lighter weightsubstrates and smaller substrates. Non-limiting examples ofradiofrequency applications include a substrate for a RF antenna, asunshield for a RF antenna, a radome, or the like. Antennas can includeflexible and/or rigid antennas, broadband planar circuited antennas(e.g. a patch antennas, an e-shaped wideband patch antenna, anelliptically polarized circular patch antenna, a monopole antenna, aplanar antenna with circular slots, a bow-tie antenna, an inverted-Fantenna and the like). In the antenna design, the circuitry can beattached to a substrate that includes the branched thermally treatedpolyamic amide aerogel and/or a mixture of the branched thermallytreated polyamic amide aerogel and other components such as otherpolymeric materials including adhesives or polymer films, organic andinorganic fibers (e.g. polyester, polyamide, polyimide, carbon, glassfibers), other organic and inorganic materials including silicaaerogels, polymer powder, glass reinforcement, etc. The use of branchedthermally treated polyamic amide aerogels in antennas enables the designsubstrates with higher throughput. In addition, the branched thermallytreated polyamic amide aerogels have coefficient of linear thermalexpansion (CTE) similar to aluminum and copper (e.g., CTE of 23 ppm/Kand 17 ppm/K), and is tunable through choice of monomer to match CTE ofother desirable materials. In some embodiments, the aerogel can be usedin sunshields and/or sunscreens used to protect RF antennas from thermalcycles due to their temperature insensitivity and RF transparency. Incertain embodiments, the aerogel can be used as a material in a radomeapplication. A radome is a structural, weatherproof enclosure thatprotects a microwave (e.g., radar) antenna. Branched thermally treatedpolyamic amide aerogels can minimize signal loss due to their lowdielectric constant and also provide structural integrity due to theirstiffness.

EXAMPLES

The present invention will be described in greater detail by way ofspecific examples. The following examples are offered for illustrativepurposes only, and are not intended to limit the invention in anymanner.

Example 1 Preparation of a Highly Branched Polyamic Acid

A reaction vessel with a mechanical stirrer and a water jacket wasemployed. The flow of the water through the reaction vessel jacket wasadjusted to maintain temperature in the range of 20-28° C. The reactionvessel was charged with dimethylsulfoxide (DMSO) (108.2 lbs. 49.1 kg),and the mechanical stirrer speed was adjusted to 120-135 rpm.1,3,5-tris(4-aminophenoxy)benzene (TAPOB, 65.03 g) was added to thesolvent. To the solution was added 4,4′-diamino-2,2′-dimethylbiphenyl(DMB, 1,080.96 g), followed by 4,4′-oxydianiline (ODA, 1,018.73 g). Afirst portion of biphenyl-tetracarboxylic acid dianhydride (BPDA)(1,524.71 g) was added. After stirring for 20 minutes, a sample of thereaction mixture was analyzed for viscosity. A second portion of BPDA(1,420.97 g) was added, and the reaction mixture was stirred for 20additional minutes. A sample of the reaction mixture was analyzed forviscosity. A third portion of BPDA (42.81 g) was added, and the reactionmixture was stirred for 20 additional minutes. A sample of the reactionmixture was analyzed for viscosity. After stirring for 8 hours, phthalicanhydride (PA, 77.62 g) was added. The resulting reaction mixture wasstirred until no more solid was visible. After 2 hours, the product wasremoved from the reaction vessel, filtered, and weighed. Structures ofthe starting materials are shown below:

Example 2 Preparation of a Highly Branched Polyamic Amide Aerogel Film

The resin (10,000 grams) prepared in Example 1 was mixed with2-methylimidazole (250 grams) for five minutes. Benzoic anhydride (945grams) was added, and the solution mixed an additional five minutes.After mixing, the resultant solution was poured onto a moving polyestersubstrate that was heated in an oven at 100° C. for 30 seconds. Thegelled film was collected and placed into an acetone bath. Afterimmersion for 24 hours, the acetone bath was exchanged for freshacetone. The soak and exchange process was repeated six times. After thefinal exchange, the gelled film was removed. The acetone solvent wasevaporated under a stream of air at room temperature, and subsequentlydried for 2 hrs hours at 200° C. The final recovered aerogel part hadopen-cell structure as observed by scanning electron microscopy (SEM)performed on a Phenom Pro Scanning Electron Microscope (Phenom-World,the Netherlands), exhibited a density of 0.20 g/cm³ and porosity of >85%as measured according to ASTM D4404-10 with a Micromeritics® AutoPore V9605 Automatic Mercury Penetrometer (Micromeritics® InstrumentCorporation, U.S.A.). The final recovered film exhibited a tensilestrength and elongation of 650 psi (4.5 MPa) and 7.6%, respectively, atroom temperature as measured according to ASTM D882-12.

Example 3 Preparation of a Highly Branched Polyamic Amide

The reaction vessel as described in Example 1 was charged withdimethylsulfoxide (DMSO) (108.2 lbs. 49.1 kg), and the mechanicalstirrer speed was adjusted to 120-135 rpm.1,3,5-tris(4-aminophenoxy)benzene (TAPOB, 65.93 g) was added to thesolvent. To the solution was added 4,4′ -diamino-2,2′ -dimethylbiphenyl(DMB, 1,081.64 g), followed by 4,4′-oxydianiline (ODA, 1,020.23 g). Afirst portion of BPDA (1,438.35 g) was added. After stirring for 20minutes, a sample of the reaction mixture was analyzed for viscosity. Asecond portion of BPDA (1,407.77 g) was added, and the reaction mixturewas stirred for 20 additional minutes. A sample of the reaction mixturewas analyzed for viscosity. A third portion of BPDA (74.35 g) was added,and the reaction mixture was stirred for 20 additional minutes. A sampleof the reaction mixture was analyzed for viscosity. After stirring for 8hours, phthalic anhydride (PA, 174.00 g) was added. The resultingreaction mixture was stirred until no more solid was visible. After 2hours, the product was removed from the reaction vessel, filtered, andweighed.

Example 4 Preparation of a Highly Branched Polyamic Amide AerogelMonolith

The resin (16.49 kilograms) prepared in Example 3 was mixed with2-methylimidazole (1.13 kilograms) for five minutes at 15 to 35° C.Benzoic anhydride (3.44 kilograms) was added, and the solution mixed anadditional five minutes. After mixing, the resultant solution was pouredinto a square 16″×16″ mold, and then left overnight at room temperature.The gelled shape was removed from the mold, and placed into an acetonebath. After immersion for 24 hours, the acetone bath was exchanged withfresh acetone. The soak and exchange process was repeated five times.After the final exchange, the gelled film part was removed. The acetonesolvent was evaporated under a stream of air at room temperature, andsubsequently dried for 1.5 hrs hours at 200° C. The final recoveredaerogel part had open-cell structure as observed by scanning electronmicroscopy (SEM) performed on a Phenom Pro Scanning Electron Microscope(Phenom-World, the Netherlands), exhibited a density of 0.14 g/cm³ andporosity of >85% as measured according to ASTM D4404-10 with aMicromeritics® AutoPore V 9605 Automatic Mercury Penetrometer(Micromeritics® Instrument Corporation, U.S.A.). The final recoveredfilm exhibited a compression strength of 230 psi (1.59 MPA) at roomtemperature (15° C. to 30° C.) as measured according to ASTM D395-14.

Example 5 Preparation of a Highly Branched Polyamic Acid

TAPOB (about 2.86 g) was added to the reaction vessel charged with about2,523.54 g DMSO as described in Example 1. To the solution was added afirst portion of DMB (about 46.75 g), followed by a first portion of ODA(about 44.09 g). After stirring for about 20 minutes, a first portion ofBPDA (about 119.46 g) was added. After stirring for about 20 minutes,TAPOB (about 2.86 g), DMB (about 46.75 g), and ODA (about 44.09 g) wereadded. After stirring for about 20 minutes, BPDA (about 119.46 g) wasadded. After stirring for about 20 minutes, TAPOB (about 2.86 g), DMB(about 46.75 g), and ODA (about 44.09 g) were added. After stirring forabout 20 minutes, (about 119.46 g) was added. After stirring for about 8hours, PA (about 50.12 g) was added. The resulting reaction mixture wasstirred until no more solids were visible. The above-steps wereperformed at a temperature of 15 to 35° C. After about 2 hours, theproduct was removed from the reaction vessel, filtered, and weighed.

Example 6 Preparation of a Highly Branched Aerogel Monolith by FreezeDrying

The resin (about 400 grams) prepared in Example 5 was mixed with2-methylimidazole (about 53.34 grams) for five minutes and then benzoicanhydride (about 161.67 grams) for five minutes. After mixing, theresultant solution was poured into a square 3″×3″ mold and placed in anoven at 75° C. for 30 minutes and then left overnight at roomtemperature. The gelled shape was removed from the mold, and placed intoan acetone bath. After immersion for 24 hours, the acetone bath wasexchanged with fresh acetone. The soak and exchange process was repeatedfive times. After the final exchange, the bath was replaced withtertiary butyl alcohol. After immersion for 24 hours, the tertiary butylalcohol bath was exchanged for fresh tertiary butyl alcohol. The soakand exchange process was repeated three times The part was subsequentlyfrozen on a shelf freezer, and subjected to subcritical drying for 96hours in at 5° C., followed by drying in vacuum at 50° C. for 48 hours.The final recovered aerogel part had open-cell structure as observed byscanning electron microscopy (SEM) performed on a Phenom Pro ScanningElectron Microscope (Phenom-World, the Netherlands), exhibited a densityof 0.15 g/cm³ and porosity of 92.2% as measured according to ASTMD4404-10 with a Micromeritics® AutoPore V 9605 Automatic MercuryPenetrometer (Micromeritics® Instrument Corporation, U.S.A.). Thedistribution of pore sizes were measured according to ASTM D4404-10using a Micromeritics® AutoPore V 9605 Automatic Mercury Penetrometer(Micromeritics® Instrument Corporation, U.S.A.), and the distribution ofpore diameters is shown in FIG. 4. Notably, and as illustrated in FIG.4, the produced aerogel includes macropores in its polymeric matrix. Italso includes mesopores in the polymeric matrix.

Example 7 Preparation of a Highly Branched Polyamic Acid

TAPOB (about 2.05 g) was added to the reaction vessel charged with about2,776.57 g DMSO as described in Example 1. To the solution was added afirst portion of DMB (about 33.54 g), followed by a first portion of ODA(about 31.63 g). After stirring for about 20 minutes, a first portion ofPMDA (about 67.04 g) was added. After stirring for about 20 minutes,TAPOB (about 2.05 g), DMB (about 33.54 g), and ODA (about 31.63 g) wereadded. After stirring for about 20 minutes, PMDA (about 67.04 g) wasadded. After stirring for about 20 minutes, TAPOB (about 2.05 g), DMB(about 33.54 g), and ODA (about 31.63 g) were added. After stirring forabout 20 minutes, PMDA (about 67.04 g) was added. After stirring forabout 8 hours, PA (about 18.12 g) was added. The resulting reactionmixture was stirred until no more solids were visible. The abovedescribed steps were performed at a temperature of 15 to 35° C. Afterabout 2 hours, the product was removed from the reaction vessel,filtered, and weighed.

Example 8 Preparation of a Highly Branched Aerogel Monolith by FreezeDrying

The resin (about 400 grams) prepared in Example 7 was mixed with2-methylimidazole (about 40.38 grams) for five minutes and then benzoicanhydride (about 122.38 grams) for five minutes at room temperature (15to 35° C.). After mixing, the resultant solution was poured into asquare 3″×3″ mold and placed in an oven at 75° C. for 30 minutes andthen left overnight at room temperature. The gelled shape was removedfrom the mold, and placed into an acetone bath. After immersion for 24hours, the acetone bath was exchanged with fresh acetone. The soak andexchange process was repeated five times. After the final exchange, thebath was replaced with tertiary butyl alcohol. After immersion for 24hours, the tertiary butyl alcohol bath was exchanged for fresh tertiarybutyl alcohol. The soak and exchange process was repeated three timesThe part was subsequently frozen on a shelf freezer, and subjected tosubcritical drying for 96 hours in at 5° C., followed by drying invacuum at 50° C. for 48 hours. The final recovered aerogel part had anopen-cell structure as observed by scanning electron microscopy (SEM)performed on a Phenom Pro Scanning Electron Microscope (Phenom-World,the Netherlands), exhibited a density of 0.23 g/cm³ and porosity of82.7% as measured according to ASTM D4404-10 with a Micromeritics®AutoPore V 9605 Automatic Mercury Penetrometer (Micromeritics®Instrument Corporation, U.S.A.). The distribution of pore sizes wasmeasured according to ASTM D4404-10 using a Micromeritics® AutoPore V9605 Automatic Mercury Penetrometer (Micromeritics® InstrumentCorporation, U.S.A.), and the distribution of pore diameters is shown inFIG. 5. Notably, and as illustrated in FIG. 5, the produced aerogelincludes macropores in its polymeric matrix. It also includes mesoporesin the polymeric matrix. From the trend in the data, it is believed thatthe aerogel contains some micropores.

Example 9 Preparation of a Linear Polyamic Acid

A reaction vessel was charged with about 776.42 g DMSO as described inExample 1. To the solution was added a first portion of ODA (about 12.76g). After stirring for about 20 minutes, a first portion of PMDA (about11.82 g) was added. After stirring for about 20 minutes, ODA (about12.76 g) was added. After stirring for about 20 minutes, PMDA (about11.82 g) was added. After stirring for about 20 minutes, ODA (about12.76 g) was added. After stirring for about 20 minutes, PMDA (about11.82 g) was added. After stirring for about 8 hours, PA (about 10.62 g)was added. The resulting reaction mixture was stirred until no moresolids were visible. The above described steps were performed at atemperature of 15 to 35° C. After about 2 hours, the product was removedfrom the reaction vessel, filtered, and weighed.

Example 10 Preparation of a Linear Aerogel Monolith by Freeze Drying

The resin (about 400 grams) prepared in Example 9 was mixed with2-methylimidazole (about 53.38 grams) for five minutes and then benzoicanhydride (about 161.80 grams) for five minutes at room temperature.After mixing, the resultant solution was poured into a square 3″×3″ moldand placed in an oven at 75° C. for 30 minutes and then left overnightat room temperature. The gelled shape was removed from the mold, andplaced into an acetone bath. After immersion for 24 hours, the acetonebath was exchanged with fresh acetone. The soak and exchange process wasrepeated five times. After the final exchange, the bath was replacedwith cyclohexane. After immersion for 24 hours, the cyclohexane bath wasexchanged for fresh cyclohexane. The soak and exchange process wasrepeated three times The part was subsequently frozen in a freezer, andsubjected to subcritical drying for 96 hours in at 5° C., followed bydrying in vacuum at 50° C. for 48 hours. The final recovered aerogelpart had open-cell structure as observed by scanning electron microscopy(SEM) performed on a Phenom Pro Scanning Electron Microscope(Phenom-World, the Netherlands), exhibited a density of 0.36 g/cm³ andporosity of 79.0% as measured according to ASTM D4404-10 with aMicromeritics® AutoPore V 9605 Automatic Mercury Penetrometer(Micromeritics® Instrument Corporation, U.S.A.). The distribution ofpore sizes was measured according to ASTM D4404-10 using aMicromeritics® AutoPore V 9605 Automatic Mercury Penetrometer(Micromeritics® Instrument Corporation, U.S.A.), and the distribution ofpore diameters is shown in FIG. 6. Notably, and as illustrated in FIG.6, the produced aerogel does not include a primarily macroporuouslystructured polymeric matrix. Rather, the matrix primarily includesmesopores in the polymeric matrix. From the trend in the data, it isbelieved that the aerogel contains some micropores.

Example 11 FTIR Data

FTIR spectral data was obtained for the Example 3 resin, the Example 4aerogel, and a comparative commercial polyimide aerogel (Kapton®polyimide aerogel from DuPont USA (Wilmington, Del., USA) by using aNicolet iS5 FT-IR spectrometer with an iD7 attenuated total reflectance(ATR) diamond crystal accessory (Thermo Scientific, Waltham, Mass.,USA). The diamond crystal was wiped down with isopropanol betweensamples. Use of the ATR allowed data collection from both solid andliquid samples with no sample preparation. The FTIR data is organized inTable 1.

TABLE 1 Comparative Commercial Absorption Vibration Example 3 Example 4Polyimide Group frequency mode Resin¹ Aerogel² Aerogel³ PolyamicAmide-Acid 2900-3200 COOH Present Absent Absent acid (PAA) and NH₂Aromatic 2900-3100 C—H Present Present Present C—H stretch Carbonyl from1710-1720 C═O Present Overlap w/ Overlap w/ acid (COOH) Imide I Imide Istretch Amide I 1660-1665 C═O Present Absent Absent (CONH) stretch AmideII 1540-1565 C—NH Present Absent Absent Carboxylate 1330-1415 COO⁻ sym.Present Overlap w/ Overlap w/ ion stretch Imide II Imide II PolyimideImide I 2900-3100 C—H stretch Present Present Present Imide I 1770-1780C═O sym. Absent Present Present stretch Imide I 1720-1740 C═O asym.Overlap Present Present stretch with amic acid Imide II 1360-1380 C═NAbsent Present Present stretch Imide III 1070-1090 C—H Overlap PresentPresent bending with DMSO Imide III 1120-1140 C—H Absent Present Presentbending Imide IV 720-740 C═O Absent Present Present bending IsoimideIsoimide 1805-1750 C═O Absent Overlap w/ Overlap w/ model Imide I ImideI compound⁴ Isoimide 1400-1425 C═N Absent Present Absent stretchIsoimide 890-905 C—O Overlap Present Absent with DMSO Amic AmideAmic-Amide 1415-1440 C═N Absent Present Absent model stretch compound⁵Amic-Amide 735-745 C—N Absent Present Absent ¹The gel/resin from Example3 was used. ²The monolith aerogel from Example 4 was used. ³Kapton ®polyimide aerogel from DuPont USA (Wilmington, DE, USA).⁴N-Phenyl-phthalisoimide was used as the isoimide model compound ascharacterized by Mochizuki et al. “Preparation and properties ofpolyisoimide as a polyimide-precursor.” Polymer journal 1994,26.3:315-323. ⁵N,N,N′,N′,-tetramethylphthalamide was used as the amicamide model compound. Model compound spectra were obtained from theNational Institute of Advanced Industrial Science and Technology (AIST)database, which can be found athttp://sdbs.db.aist.go.jp/sdbs/cgi-bin/direct_frame_top.cgi.

Example 12 Heat Treating of Example 4 Aerogel

The aerogel of Example 4 was heated at about 300° C. for about 2 hoursin an argon atmosphere. The final recovered aerogel part had open-cellstructure as observed by scanning electron microscopy (SEM) performed ona Phenom Pro Scanning Electron Microscope (Phenom-World, theNetherlands), exhibited a density of 0.20 g/cm³ and porosity of >85% asmeasured according to ASTM D4404-10 with a Micromeritics® AutoPore V9605 Automatic Mercury Penetrometer (Micromeritics® InstrumentCorporation, U.S.A.). The final recovered film exhibited a compressionstrength of 270 psi at room temperature as measured according to ASTMD395-14. The final recovered part exhibited a Total Mass Loss value of0.22% when measured according to ASTM E595-15, and a Collected VolatileCondensable Material value of 0.03%.

Example 13 Determination of Polyamic Amide Incorporation in the Aerogelsof Example 3

The resin (74 grams) prepared in Example 3 was mixed with4-(4-fluorophenyl)-1H-imidazole (10 grams) for three minutes. Benzoicanhydride (15.4 grams) was added, and the solution mixed an additionalthree minutes. After mixing, the resultant solution was poured into3″×3″ fluoropolymer coated aluminum mold at room temperature. After 24hours, the gel was collected and placed into an acetone bath. Afterimmersion for 24 hours, the acetone bath was exchanged for freshacetone. The soak and exchange process was repeated six times. After thefinal exchange, the gelled shape was removed. The acetone solvent wasevaporated at room temperature. Samples were taken after air drying,after drying for 90 minutes at 200° C. in air, after drying for 60minutes at 300° C. under flowing argon, and after drying for 300° C. 12hours under dynamic vacuum. The fluorine content was analyzed viaelemental analysis. The results are listed in Table 2.

TABLE 2 Corresponding Weight Percent Drying Profile % Fluorine Poly(AmicAmide)* Air Drying Only 0.23% 6.4% 200° C. in Air 90 Minutes 0.17% 4.8%300° C. in Argon 60 Minutes 0.10% 2.8% 300° C. in Vacuum 12 Hours 0.10%2.8% *Calculated based on the formula weight of 2-methylimidazole.

1. A method of making, a thermally treated polyamic amide aerogel, themethod comprising: (a) providing at least one diamine compound to asolvent to form a solution; (b) providing at least one dianhydridecompound to the solution of step (a) under conditions sufficient to forma polyamic acid solution; (c) providing a secondary amine to thepolyamic acid solution; (d) subjecting the solution of step (c) toconditions suitable to produce a polymer matrix solution comprising apolyamic amide; (e) subjecting the polymer matrix solution to conditionssufficient to form an aerogel comprising an open-cell structured polymermatrix having a polyamic amide; and (f) thermally treating the (e)polyamic amide aerogel at a temperature sufficient to lower the amountof the polyamic amide in the aerogel.
 2. The method of claim 1, whereinstep (f) conditions comprise heating the aerogel at a temperature of275° C. to 550° C., or 290° C. to 500° C., or 300° C. to 350° C., toproduce a thermally treated polyimide aerogel.
 3. The method of claim 2,wherein step (f) is performed under an inert atmosphere or in air. 4.The method of claim 1, further comprising subjecting the thermallytreated polyimide aerogel to a second temperature cycle, under vacuum orin air to remove compounds not chemically bound to the polymer matrix.5. The method of claim 1, wherein the aerogel includes at least 0.01 wt.% and up to 4.95 wt. % of the polyamic amide polymer.
 6. The method ofclaim 1, wherein thermally treating reduces the amount of polyamic amidein the aerogel by at least 15%, or at least 50%.
 7. The method of claim1, wherein the secondary amine is a substituted or an unsubstitutedcyclic amine, a substituted or an unsubstituted aromatic amine, orcombinations thereof.
 8. The method of claim 1, wherein secondary aminefurther comprises at least one secondary nitrogen and at least onetertiary nitrogen.
 9. The method of claim 8, wherein the secondary amineis imidazole or a substituted imidazole, a triazole or a substitutedtriazole, a tetrazole or substituted tetrazole, a purine or asubstituted purine, a pyrazole or a substituted pyrazole, orcombinations thereof.
 10. The method of claim 9, wherein the nitrogenatoms are separated by at least one carbon atom.
 11. The method of claim1, wherein the secondary amine in step (c) has the following generalstructure:

where R₃, R₄, and R₅ are each individually a hydrogen, an alkyl group,or a substituted alkyl group, or an aromatic group or a substitutedgroup, or R₄, and R₅ come together with other atoms to form a cyclicstructure.
 12. The method of claim 11, wherein the alkyl group has 1 to12 carbon atoms, 2 to 6 carbon atoms, 3 to 8 carbon atoms, 5 to 12carbon atoms, preferably 1 to 6 carbon atoms.
 13. The method of claim11, wherein R₃ is a methyl group or an ethyl group, and R₄ and R₅ are Hatoms, an alkyl group, or a substituted alkyl.
 14. The method of claim13, wherein R₃ is a methyl group and R₄ and R₅ are H atoms.
 15. Themethod of claim 14, wherein R₃ is an ethyl group and R₄ and R₅ are eachindividually a H atom, an alkyl group, or a substituted alkyl,preferably, R₄ is a methyl group and R₅ is a H atom.
 16. The method ofclaim 1, wherein step (d) comprises providing a dehydrating agent priorto, during, or after, adding the secondary amine.
 17. The method ofclaim 1, wherein step (e) comprises forming a gel from the solution andremoving the solvent from the gel.
 18. The method of claim 17,comprising subjecting the gel to a drying step to remove the solvent.19. The method of claim 18, wherein the drying step comprisessupercritical drying, subcritical drying, thermal drying, evaporativeair drying, vacuum drying, or any combination thereof.
 20. The method ofclaim 19, wherein drying comprises evaporative air drying.
 21. Themethod of claim 20, wherein the drying step comprises: (i) subjectingthe gel to conditions sufficient to freeze the solvent to form a frozenmaterial; and (ii) subjecting the frozen material to a subcriticaldrying step sufficient to form an open-cell structure.
 22. The method ofclaim 1, further comprising subjecting the gel to at least one solventexchange with a different solvent prior to drying the gel.
 23. Themethod of claim 22, wherein at least one solvent exchange is performedwith acetone.
 24. A thermally treated polyamic amide aerogel comprisingan open-cell structured polymer matrix that includes a polyamic amidepolymer in an amount less than the polyamic amide polymer prior tothermal treatment.
 25. The aerogel of claim 24, wherein the aerogel,when exposed to heat, does not produce a gas. 26-41. (canceled)
 42. Theaerogel of claim 24, wherein the aerogel includes at 0.01 wt. % to up to6 wt. % of the polyamic amide polymer. 43-45. (canceled)
 46. An articleof manufacture comprising the aerogel of claim
 24. 47-51. (canceled)